A hat-trick from Carlos Vela, a brace from Nicklas Bendtner and another from Jack Wilshere was enough to dispose of Kevin Blackwell's men.
Arsenal began the brightest and controlled the game throughout despite Arsene Wenger picking his youngest ever squad.
However, the first effort that hit its intended target came from United's Danny Webber on 10 minutes.
His mazy run past a couple of Arsenal defenders looked promising, but his eventual shot was too tame to trouble Lukasz Fabianski.
That was the only time the Polish keeper and captain for the evening was required.
Paddy Kenny was forced into his first save of the game on 25 minutes when a slick move from the Gunners culminated with a shot on the turn from inside the area by the impressive Johan Djourou that the Blades keeper parried for a corner.
Arsenal opened the scoring on 31 minutes when Bendtner was fed by Vela and ran at the defence before curling a low shot into the right corner of the net.
The Denmark striker doubled his and Arsenal's tally on 43 minutes when Aaron Ramsey found Bendtner with a clever back heel that unlocked United's offside trap and the striker coolly slotted beyond Kenny.
On the stroke of half-time the Gunners went three up when Vela scored his first when released from the right flank by Wilshere to spring into the area before curling a left-foot shot around the helpless goalkeeper.
Vela scored his second just three minutes after the interval when he took a pass forward from Kieran Gibbs on his chest before turning to run at goal and then casually lifting the ball over the flat-footed Blades goalkeeper.
Then 16-year old sensation Wilshere also got in on the act to score Arsenal's fifth just before the hour when he received a pass from Perez Fran Merida on the right side of the United area and the youngster drilled a low drive into the bottom right corner of the goal that Kenny had no chance of reaching.
Arsenal continued to pour forward and looked like scoring any time and the victory was sealed with an 87th minute hat-trick by Vela, whose crisp left-foot shot from inside the left side of the area nestled in the right corner of the net before Kenny could even react.