Bobby Boswell and Rob Vincent added goals to help United (11-9-13) win its fourth straight. City (14-10-9) was eliminated from the Supporters' Shield race and fell three points behind the New York Red Bulls for the top seed in the East.
United opened the scoring in the 27th minute when Lloyd Sam sent in a low cross and Luciano Acosta deflected it through to Mullins for the finish. Boswell headed home Sam's corner in the 31st and Vincent capitalized on a defensive miscue in the 45th.
David Villa scored for NYCFC on a penalty kick in the 72nd minute - his 22nd of the season. But he dropped a goal behind Bradley Wright-Phillips in the Golden Boot standings after Wright-Phillips added two goals to his tally Sunday.