For the Gunners it was a third league defeat on the bounce and leaves them 18 points behind City at the summit and struggling even to clinch a top fourth berth and Champions League qualification.
Considering the history between these two teams the first half was uneventful with United looking the brighter and deserving their lead given to them by Antonio Valencia just before the break as he headed home Ryan Giggs cross.
Arsenal improved in the second half and after Robin van Persie had missed one golden opportunity to level the Dutchman made no mistake when opportunity knocked again and he scored his 19th league goal of the season.
Valencia turned provider this time, and after exchanging passes with substitute Park Ji-Sung his pass found Danny Welbeck who finished emphatically ensuring United continue to have the best away record in the top flight.
Referee: Mike Dean
Venue: Emirates Stadium
Yellow cards: Koscienly (ARS), Rosicky (ARS), Ramsey (ARS), Song (ARS), Van Persie (ARS), Evra (MANU), Rafael (MANU)