UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 25)*

American Midstream Partners, LP

(Name of Issuer)

Common Units Representing Limited Partner Interests

(Title of Class of Securities)

02752P 100

(CUSIP Number)

Christine Miller

Magnolia Infrastructure Partners, LLC

c/o ArcLight Capital Partners

200 Clarendon Street, 55th Floor

Boston, MA 02117

(617) 531-6338

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

January 2, 2019

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D and is filing this Schedule because of Rule 13d-1(e), Rule 13d-1(f) or 13d-1(g), check the following box:  ☐

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act.

Note 1: Represents 1,349,609 common units of the Issuer (“Common Units”) held by American Midstream GP, LLC, the Issuer’s general partner.

Note 2: Based on 53,816,114 Common Units outstanding, which equals the sum of (a) 53,005,627 Common Units outstanding as of November 6, 2018 and (b) 810,517 Common Units issued to Magnolia Infrastructure Holdings, LLC on December 10, 2018.

Note 1: Represents 1,349,609 common units of the Issuer (“Common Units”) held by American Midstream GP, LLC, which is approximately 23% owned by AMID GP Holdings, LLC.

Note 2: Based on 53,816,114 Common Units outstanding, which equals the sum of (a) 53,005,627 Common Units outstanding as of November 6, 2018 and (b) 810,517 Common Units issued to Magnolia Infrastructure Holdings, LLC on December 10, 2018.

Note 1: Represents 7,707,571 Series A-l Convertible Preferred Units (“Series A-l Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 9,874,169 common units of the Issuer (“Common Units”) and 1,349,609 Common Units held by American Midstream GP, LLC, which is approximately 77% owned by High Point.

Note 2: Based on 63,690,313 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 53,005,627 Common Units outstanding as of November 6, 2018, (b) 810,517 Common Units issued to Magnolia Infrastructure Holdings, LLC on December 10, 2018 and (c) 9,874,169 Common Units issuable upon the conversion of Series A-l Units outstanding.

Note 1: Represents 7,707,571 Series A-1 Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 9,874,169 common units of the Issuer (“Common Units”), which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 3,302,158 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 4,230,395 Common Units, 1,349,609 Common Units held by American Midstream GP, LLC, which is approximately 77% owned by High Point, and 618,921 Common Units held by Magnolia.

Note 2: Based on 67,920,708 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 53,005,627 Common Units outstanding as of November 6, 2018, (b) 810,517 Common Units issued to Magnolia Infrastructure Holdings, LLC on December 10, 2018, (c) 9,874,169 Common Units issuable upon the conversion of Series A-l Units outstanding and (d) 4,230,395 Common Units issuable upon the conversion of Series A-2 Units outstanding.

Note 1: Represents 7,707,571 Series A-l Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 9,874,169 common units of the Issuer (“Common Units”), which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 3,302,158 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 4,230,395 Common Units, 9,241,642 Series C Convertible Preferred Units (“Series C Units”) held by Magnolia Infrastructure Holdings, LLC (“Magnolia Holdings”), convertible into 9,254,580 Common Units, 1,291,869 Common Units issuable upon exercise of the warrant issued to Magnolia Holdings by American Midstream Partners, LP, dated April 25, 2016, 10,563,942 Common Units held by Magnolia Holdings, 1,349,609 Common Units held by American Midstream GP, LLC, which is approximately 77% owned by High Point and approximately 23% owned by AMID GP Holdings, LLC, which is approximately 93% owned by Magnolia Holdings, and 618,921 Common Units held by Magnolia, taking into account the transactions discussed in Item 3.

Note 2: Based on 78,467,157 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 53,005,627 Common Units outstanding as of November 6, 2018, (b) 810,517 Common Units issued to Magnolia Holdings on December 10, 2018, (c) 9,874,169 Common Units issuable upon the conversion of Series A-1 Units outstanding, (d) 4,230,395 Common Units issuable upon the conversion of Series A-2 Units outstanding, (e) 9,254,580 Common Units issuable upon the conversion of Series C Units outstanding and (f) 1,291,869 Common Units issuable upon exercise of the warrant issued to Magnolia Holdings by American Midstream Partners, LP, dated April 25, 2016.

Note 1: Represents 7,707,571 Series A-l Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 9,874,169 common units of the Issuer (“Common Units”), which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 3,302,158 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 4,230,395 Common Units, 9,241,642 Series C Convertible Preferred Units (“Series C Units”) held by Magnolia Infrastructure Holdings, LLC (“Magnolia Holdings”), convertible into 9,254,580 Common Units, 1,291,869 Common Units issuable upon exercise of the warrant issued to Magnolia Holdings by American Midstream Partners, LP, dated April 25, 2016, 10,563,942 Common Units held by Magnolia Holdings, 1,349,609 Common Units held by American Midstream GP, LLC, which is approximately 77% owned by High Point and approximately 23% owned by AMID GP Holdings, LLC, which is approximately 93% owned by Magnolia Holdings, 618,921 Common Units held by Magnolia and 2,853,482 Common Units held by Busbar II, LLC, taking into account the transactions discussed in Item 3.

Note 2: Based on 78,467,157 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 53,005,627 Common Units outstanding as of November 6, 2018, (b) 810,517 Common Units issued to Magnolia Holdings on December 10, 2018, (c) 9,874,169 Common Units issuable upon the conversion of Series A-1 Units outstanding, (d) 4,230,395 Common Units issuable upon the conversion of Series A-2 Units outstanding, (e) 9,254,580 Common Units issuable upon the conversion of Series C Units outstanding and (f) 1,291,869 Common Units issuable upon exercise of the warrant issued to Magnolia Holdings by American Midstream Partners, LP, dated April 25, 2016.

Note 1: Represents 7,707,571 Series A-l Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 9,874,169 common units of the Issuer (“Common Units”), which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 3,302,158 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 4,230,395 Common Units, 9,241,642 Series C Convertible Preferred Units (“Series C Units”) held by Magnolia Infrastructure Holdings, LLC (“Magnolia Holdings”), convertible into 9,254,580 Common Units, 1,291,869 Common Units issuable upon exercise of the warrant issued to Magnolia Holdings by American Midstream Partners, LP, dated April 25, 2016, 10,563,942 Common Units held by Magnolia Holdings, 1,349,609 Common Units held by American Midstream GP, LLC, which is approximately 77% owned by High Point and approximately 23% owned by AMID GP Holdings, LLC, which is approximately 93% owned by Magnolia Holdings, 618,921 Common Units held by Magnolia and 2,853,482 Common Units held by Busbar II, LLC, taking into account the transactions discussed in Item 3.

Note 2: Based on 78,467,157 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 53,005,627 Common Units outstanding as of November 6, 2018, (b) 810,517 Common Units issued to Magnolia Holdings on December 10, 2018, (c) 9,874,169 Common Units issuable upon the conversion of Series A-1 Units outstanding, (d) 4,230,395 Common Units issuable upon the conversion of Series A-2 Units outstanding, (e) 9,254,580 Common Units issuable upon the conversion of Series C Units outstanding and (f) 1,291,869 Common Units issuable upon exercise of the warrant issued to Magnolia Holdings by American Midstream Partners, LP, dated April 25, 2016.

Note 1: Represents 7,707,571 Series A-l Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 9,874,169 common units of the Issuer (“Common Units”), which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 3,302,158 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 4,230,395 Common Units, 9,241,642 Series C Convertible Preferred Units (“Series C Units”) held by Magnolia Infrastructure Holdings, LLC (“Magnolia Holdings”), convertible into 9,254,580 Common Units, 1,291,869 Common Units issuable upon exercise of the warrant issued to Magnolia Holdings by American Midstream Partners, LP, dated April 25, 2016, 10,563,942 Common Units held by Magnolia Holdings, 1,349,609 Common Units held by American Midstream GP, LLC, which is approximately 77% owned by High Point and approximately 23% owned by AMID GP Holdings, LLC, which is approximately 93% owned by Magnolia Holdings, 618,921 Common Units held by Magnolia and 2,853,482 Common Units held by Busbar II, LLC, taking into account the transactions discussed in Item 3.

Note 2: Based on 78,467,157 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 53,005,627 Common Units outstanding as of November 6, 2018, (b) 810,517 Common Units issued to Magnolia Holdings on December 10, 2018, (c) 9,874,169 Common Units issuable upon the conversion of Series A-1 Units outstanding, (d) 4,230,395 Common Units issuable upon the conversion of Series A-2 Units outstanding, (e) 9,254,580 Common Units issuable upon the conversion of Series C Units outstanding and (f) 1,291,869 Common Units issuable upon exercise of the warrant issued to Magnolia Holdings by American Midstream Partners, LP, dated April 25, 2016.

Note 1: Represents 7,707,571 Series A-l Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 9,874,169 common units of the Issuer (“Common Units”), which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 3,302,158 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 4,230,395 Common Units, 9,241,642 Series C Convertible Preferred Units (“Series C Units”) held by Magnolia Infrastructure Holdings, LLC (“Magnolia Holdings”), convertible into 9,254,580 Common Units, 1,291,869 Common Units issuable upon exercise of the warrant issued to Magnolia Holdings by American Midstream Partners, LP, dated April 25, 2016, 10,563,942 Common Units held by Magnolia Holdings, 1,349,609 Common Units held by American Midstream GP, LLC, which is approximately 77% owned by High Point and approximately 23% owned by AMID GP Holdings, LLC, which is approximately 93% owned by Magnolia Holdings, 618,921 Common Units held by Magnolia and 2,853,482 Common Units held by Busbar II, LLC, taking into account the transactions discussed in Item 3.

Note 2: Based on 78,467,157 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 53,005,627 Common Units outstanding as of November 6, 2018, (b) 810,517 Common Units issued to Magnolia Holdings on December 10, 2018, (c) 9,874,169 Common Units issuable upon the conversion of Series A-1 Units outstanding, (d) 4,230,395 Common Units issuable upon the conversion of Series A-2 Units outstanding, (e) 9,254,580 Common Units issuable upon the conversion of Series C Units outstanding and (f) 1,291,869 Common Units issuable upon exercise of the warrant issued to Magnolia Holdings by American Midstream Partners, LP, dated April 25, 2016.

Note 1: Represents 7,707,571 Series A-l Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 9,874,169 common units of the Issuer (“Common Units”), which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 3,302,158 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 4,230,395 Common Units, 9,241,642 Series C Convertible Preferred Units (“Series C Units”) held by Magnolia Infrastructure Holdings, LLC (“Magnolia Holdings”), convertible into 9,254,580 Common Units, 1,291,869 Common Units issuable upon exercise of the warrant issued to Magnolia Holdings by American Midstream Partners, LP, dated April 25, 2016, 10,563,942 Common Units held by Magnolia Holdings, 1,349,609 Common Units held by American Midstream GP, LLC, which is approximately 77% owned by High Point and approximately 23% owned by AMID GP Holdings, LLC, which is approximately 93% owned by Magnolia Holdings, 618,921 Common Units held by Magnolia and 2,853,482 Common Units held by Busbar II, LLC, taking into account the transactions discussed in Item 3.

Note 2: Based on 78,467,157 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 53,005,627 Common Units outstanding as of November 6, 2018, (b) 810,517 Common Units issued to Magnolia Holdings on December 10, 2018, (c) 9,874,169 Common Units issuable upon the conversion of Series A-1 Units outstanding, (d) 4,230,395 Common Units issuable upon the conversion of Series A-2 Units outstanding, (e) 9,254,580 Common Units issuable upon the conversion of Series C Units outstanding and (f) 1,291,869 Common Units issuable upon exercise of the warrant issued to Magnolia Holdings by American Midstream Partners, LP, dated April 25, 2016.

This Amendment No. 25 to Schedule 13D amends and supplements the Statement on Schedule 13D filed on May 3, 2013, as amended by Amendment No. 1 filed on November 14, 2014, Amendment No. 2 filed on February 17, 2015, Amendment No. 3 filed on April 3, 2015, Amendment No. 4 filed on May 15, 2015, Amendment No. 5 filed on August 18, 2015, Amendment No. 6 filed on August 19, 2015, Amendment No. 7 filed on September 17, 2015, Amendment No. 8 filed on November 18, 2015, Amendment No. 9 filed on December 22, 2015, Amendment No. 10 filed on January 6, 2016, Amendment No. 11 filed on January 11, 2016, Amendment No. 12 filed on February 1, 2016, Amendment No. 13 filed on February 24, 2016, Amendment No. 14 filed on April 27, 2016, Amendment No. 15 filed on May 18, 2016, Amendment No. 16 filed on November 3, 2016, Amendment No. 17 filed on December 6, 2016, Amendment No. 18 filed on March 8, 2017, Amendment No. 19 filed on August 18, 2017, Amendment No. 20 filed on October 12, 2017, Amendment No. 21 filed on August 20, 2018, Amendment No. 22 filed on September 28, 2018, Amendment No. 23 filed on December 6, 2018 and Amendment No. 24 filed on December 11, 2018 (as amended, this “Schedule 13D”), filed with respect to the common units representing limited partner interests (“Common Units”) of American Midstream Partners, LP (the “Issuer”).

Item 4 is hereby amended and supplemented by adding the following immediately after the last paragraph:

On January 2, 2019, Magnolia Infrastructure Holdings, LLC (“Magnolia Holdings”) delivered a non-binding offer revising the Offer Letter (the “Revised Offer Letter”) to members of the conflicts committee of the board of directors of American Midstream GP, LLC to acquire all of the issued and outstanding publicly held Common Units of the Issuer that are not directly owned by Magnolia Holdings or any of its affiliates in exchange for $4.50 in cash for each such Common Unit. The foregoing description of the Revised Offer Letter does not purport to be complete and is qualified in its entirety by reference to the full text of the Revised Offer Letter, which is filed as Exhibit 1 hereto and is incorporated by reference in its entirety into this Item 4.

See the Exhibit Index following the signature pages hereto.

SIGNATURES

After reasonable inquiry and to the best of each of the undersigned’s knowledge and belief, each of the undersigned hereby certifies that the information set forth in this statement is true, complete and correct.

Dated: January 3, 2019

EXHIBIT INDEX