A day after their senior side collected the league trophy for a record 19th time with highly rated Ravel Morrison and Will Keane each scoring twice.
Sir Alex Ferguson was in the stands to witness the historic triumph against a talented group of young Blades players.
They pegged United back twice in the first leg but were outplayed on Monday.
Sheffield-born Jordan Slew, who has already made eight first-team appearances and has a couple of England under-19 caps to his name, impressed for the visitors.
He went close on a couple of occasions before the break, including a dipping shot that had Sam Johnstone scrambling across his goal to watch it to safety.
For Manchester United - for whom Sir Matt Busby's 'Babes' won the Youth Cup five years on the trot from 1953 - French import Paul Pogba, Ryan Tunnicliffe, Morrison and Keane all shone brightly.
But Morrison was the key player - securing his brace through a smart piece of control and finish in the first half and a run and low shot from 20 yards in the second.
With Keane scoring from the spot and adding his own second, United were ultimately comfortable winners, rendering Joe Ironside's strike for the Blades nothing more than a consolation.
Manchester United became the first club to lift the FA Youth Cup 10 times after beating Sheffield United 4-1 to seal a 6-3 final win on aggregate.