In a highly entertaining clash in front of a packed house, United led 2-0 at half-time after goals from Mason Greenwood and Marcus Rashford, both products of the club's academy.
Ole Gunnar Solskjaer made 11 changes at the break, and Phil Jones netted early in the second half with a powerful header from Andreas Pereira's corner. Anthony Martial then converted a penalty in the 69th minute to complete the rout.
United's tour continues on Saturday when they face Inter Milan in Singapore in the International Champions Cup.
It was a clinical display from United, who looked dangerous from the off when Rashford hit the post in the second minute.
Paul Pogba was at the heart of everything for the Red Devils in the first half, and he had a key hand in Greenwood's seventh-minute opener.
The Frenchman fed Aaron Wan-Bissaka brilliantly on the right, the new signing squared across the box, and the 17-year-old Greenwood turned home.
Leeds were slightly shellshocked but eventually came into the game when Jack Harrison rippled the side netting in the 21st minute before Sergio Romero saved from Pablo Hernandez just before the half hour.
From the resultant counter-attack, though, United got their second.
Scott McTominay surged through the middle, made the right call in feeding Rashford on the left, and the Englishman feinted to create space in the box before opening his body up and finding the corner of the net with his right foot:
Moments before the break, another fantastic move involving Pogba and Rashford set up Daniel James for an effort that hit the post.
Wholesale changes during half-time made for a different rhythm in the second half:
But United were similarly dominant, and Jones' powerful header to make it 3-0 in the 51st minute came after Pereira's fizzing long-range effort was tipped over the bar by Leeds goalkeeper Kiko Casilla:
Tahith Chong then won a penalty in the 68th minute after being felled in the box by Liam Cooper, who was bamboozled by the 19-year-old's footwork.
Martial stepped up and cooly converted into the bottom corner to complete a satisfying night for Solskjaer and his squad.