UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 10)*

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 6, 2016

(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 remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities and for any subsequent amendment containing information which would alter disclosures provided in a prior cover page.

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 6,447,957 Series A-1 Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 7,077,679 Common Units, which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 2,762,503 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 3,032,295 Common Units, 1,349,609 Series B Convertible Preferred Units (“Series B Units”) held by American Midstream GP, LLC, which is 95% owned by High Point, and 618,921 Common Units held by Magnolia.

Note 2: Based on 41,885,412 common units of the Issuer (“Common Units”) outstanding, on an as-converted basis, which equals the sum of (a) 30,425,829, the number of Common Units outstanding as of November 6, 2015, as reported in the Issuer’s Quarterly Report on Form 10-Q filed on November 9, 2015, (b) 7,077,679 Common Units issuable upon the conversion of Series A-1 Units outstanding as of November 13, 2015, (c) 3,032,295 Common Units issuable upon the conversion of Series A-2 Units outstanding as of November 13, 2015 and (d) 1,349,609 Common Units issuable upon the conversion of Series B Units outstanding as of November 13, 2015.

Note 1: Represents 6,447,957 Series A-1 Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 7,077,679 Common Units, which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”) and 1,349,609 Series B Convertible Preferred Units (“Series B Units”) held by American Midstream GP, LLC (the “General Partner”), which is 95% owned by High Point.

Note 2: Based on 38,853,117 common units of the Issuer (“Common Units”) outstanding, on an as-converted basis, which equals the sum of (a) 30,425,829, the number of Common Units outstanding as of November 6, 2015, as reported in the Issuer’s Quarterly Report on Form 10-Q filed on November 9, 2015, (b) 7,077,679 Common Units issuable upon the conversion of Series A-1 Units outstanding as of November 13, 2015 and (c) 1,349,609 Common Units issuable upon the conversion of Series B Units outstanding as of November 13, 2015.

Note 1: Represents 1,349,609 Series B Convertible Preferred Units (“Series B Units”) held by American Midstream GP, LLC, the Issuer’s general partner (the “General Partner”).

Note 2: Based on 31,775,438 common units of the Issuer (“Common Units”) outstanding, on an as-converted basis, which equals the sum of (a) 30,425,829, the number of Common Units outstanding as of November 6, 2015, as reported in the Issuer’s Quarterly Report on Form 10-Q filed on November 9, 2015 and (b) 1,349,609 Common Units issuable upon the conversion of Series B Units outstanding as of November 13, 2015.

Note 1: Represents 6,447,957 Series A-1 Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 7,077,679 Common Units, which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 2,762,503 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 3,032,295 Common Units, 1,349,609 Series B Convertible Preferred Units (“Series B Units”) held by American Midstream GP, LLC, which is 95% owned by High Point, 618,921 Common Units held by Magnolia, and 371,972 Common Units held by Busbar II, LLC.

Note 2: Based on 41,855,412 common units of the Issuer (“Common Units”) outstanding, on an as-converted basis, which equals the sum of (a) 30,425,829, the number of Common Units outstanding as of November 6, 2015, as reported in the Issuer’s Quarterly Report on Form 10-Q filed on November 9, 2015, (b) 7,077,679 Common Units issuable upon the conversion of Series A-1 Units outstanding as of November 13, 2015, (c) 3,032,295 Common Units issuable upon the conversion of Series A-2 Units outstanding as of November 13, 2015 and (d) 1,349,609 Common Units issuable upon the conversion of Series B Units outstanding as of November 13, 2015.

Note 1: Represents 6,447,957 Series A-1 Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 7,077,679 Common Units, which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 2,762,503 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 3,032,295 Common Units, 1,349,609 Series B Convertible Preferred Units (“Series B Units”) held by American Midstream GP, LLC, which is 95% owned by High Point, 618,921 Common Units held by Magnolia, and 371,972 Common Units held by Busbar II, LLC.

Note 2: Based on 41,885,412 common units of the Issuer (“Common Units”) outstanding, on an as-converted basis, which equals the sum of (a) 30,425,829, the number of Common Units outstanding as of November 6, 2015, as reported in the Issuer’s Quarterly Report on Form 10-Q filed on November 9, 2015, (b) 7,077,679 Common Units issuable upon the conversion of Series A-1 Units outstanding as of November 13, 2015, (c) 3,032,295 Common Units issuable upon the conversion of Series A-2 Units outstanding as of November 13, 2015 and (d) 1,349,609 Common Units issuable upon the conversion of Series B Units outstanding as of November 13, 2015.

Note 1: Represents 6,447,957 Series A-1 Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 7,077,679 Common Units, which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 2,762,503 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 3,032,295 Common Units, 1,349,609 Series B Convertible Preferred Units (“Series B Units”) held by American Midstream GP, LLC, which is 95% owned by High Point, 618,921 Common Units held by Magnolia, and 371,972 Common Units held by Busbar II, LLC.

Note 2: Based on 41,885,412 common units of the Issuer (“Common Units”) outstanding, on an as-converted basis, which equals the sum of (a) 30,425,829, the number of Common Units outstanding as of November 6, 2015, as reported in the Issuer’s Quarterly Report on Form 10-Q filed on November 9, 2015, (b) 7,077,679 Common Units issuable upon the conversion of Series A-1 Units outstanding as of November 13, 2015, (c) 3,032,295 Common Units issuable upon the conversion of Series A-2 Units outstanding as of November 13, 2015 and (d) 1,349,609 Common Units issuable upon the conversion of Series B Units outstanding as of November 13, 2015.

Note 1: Represents 6,447,957 Series A-1 Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 7,077,679 Common Units, which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 2,762,503 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 3,032,295 Common Units, 1,349,609 Series B Convertible Preferred Units (“Series B Units”) held by American Midstream GP, LLC, which is 95% owned by High Point, 618,921 Common Units held by Magnolia, and 371,972 Common Units held by Busbar II, LLC.

Note 2: Based on 41,885,412 common units of the Issuer (“Common Units”) outstanding, on an as-converted basis, which equals the sum of (a) 30,425,829, the number of Common Units outstanding as of November 6, 2015, as reported in the Issuer’s Quarterly Report on Form 10-Q filed on November 9, 2015, (b) 7,077,679 Common Units issuable upon the conversion of Series A-1 Units outstanding as of November 13, 2015, (c) 3,032,295 Common Units issuable upon the conversion of Series A-2 Units outstanding as of November 13, 2015 and (d) 1,349,609 Common Units issuable upon the conversion of Series B Units outstanding as of November 13, 2015.

Note 1: Represents 6,447,957 Series A-1 Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 7,077,670 Common Units, which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 2,762,503 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 3,032,295 Common Units, 1,349,609 Series B Convertible Preferred Units (“Series B Units”) held by American Midstream GP, LLC, which is 95% owned by High Point, 618,921 Common Units held by Magnolia, and 371,972 Common Units held by Busbar II, LLC.

Note 2: Based on 41,885,412 common units of the Issuer (“Common Units”) outstanding, on an as-converted basis, which equals the sum of (a) 30,425,829, the number of Common Units outstanding as of November 6, 2015, as reported in the Issuer’s Quarterly Report on Form 10-Q filed on November 9, 2015, (b) 7,077,679 Common Units issuable upon the conversion of Series A-1 Units outstanding as of November 13, 2015, (c) 3,032,295 Common Units issuable upon the conversion of Series A-2 Units outstanding as of November 13, 2015 and (d) 1,349,609 Common Units issuable upon the conversion of Series B Units outstanding as of November 13, 2015.

This Amendment No. 10 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, and Amendment No. 9 filed on December 22, 2015 (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 3 is hereby amended by adding the following at the end hereof:

In various open market purchases between December 22, 2015 and January 6, 2016, Busbar II, LLC, a direct, wholly owned subsidiary of ArcLight Energy Partners Fund V, L.P., acquired an aggregate of 371,972 of the Issuer’s outstanding Common Units for a total of $2,781,607.86. The purchase of such securities was funded from cash on hand of the Reporting Persons.

Item 5(c) of the Schedule 13D is hereby amended and restated as follows:

(c) The transactions effected by the Reporting Persons during the past 60 days are set forth on Schedule I attached hereto.

See the Exhibit Index following the signature pages hereto.

SIGNATURES

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Dated: January 7, 2016

EXHIBIT INDEX

SCHEDULE I

Recent Open Market Transactions by Reporting Persons in the Securities of the Issuer

The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth above.