Manchester City's Yaya Toure celebrates after scoring during their FA Cup semi-final against Man United
Toure netted in the 52nd minute at Wembley Stadium after poor clearances from both United goalkeeper Edwin van der Sar and midfielder Michael Carrick.
United was reduced to 10 men in the 72nd when Paul Scholes was sent off for a dangerous, high challenge on Pablo Zabaleta.
But City had been overrun for most of the first half and Dimitar Berbatov should have put United ahead when he lofted the ball over from close range in front of an unguarded net.
City hasn't won a trophy since the 1976 League Cup.