13D Filing: Magnolia Infrastructure Holdings, LLC and American Midstream Partners LP (AMID)

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 20)*

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)

October 2, 2017

(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 persons 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.