Steven Gerrard's penalty put Liverpool ahead, but Wayne Rooney and Juan Mata scored within two minutes of each other before Jesse Lingard rounded off the 3-1 win at the death.
The victory, which came in front of 51,104 rain-soaked fans at the uncovered Sun Life Stadium in Miami, meant United lifted the International Champions Cup - the pre-season tournament that contained eight of Europe's best teams.
The trophy, and the Chevrolet Cup, which Van Gaal won after United beat the Los Angeles Galaxy a fortnight ago, will not take pride of place in the Dutchman's packed cabinet.
But the former Barcelona and Bayern Munich manager will at least be happy to go back to Manchester with five wins under his belt and an enhanced knowledge of his squad.
United rode their luck at times in Miami - they could not handle Raheem Sterling for most of the night - but overall on this tour, Van Gaal's men have performed very well.
Rooney, who bagged four goals in the US, and Mata have been two of the best performers, along with Ander Herrera, Jonny Evans and Darren Fletcher.
The players have reacted well to Van Gaal's training techniques and the 3-4-1-2 system he has used has had success against the Galaxy, Roma, Inter Milan, Real Madrid and now Liverpool.
More importantly, the atmosphere around the camp is in complete contrast to the edgy feeling that lingered around the team under David Moyes.
Despite 24 hours of torrential rain in Florida, organisers attempted to raise the spirits of the fans with a flurry of pre-match fireworks.
Once the smoke had faded the match began, and United were on top early on.
They should have gone ahead when Ashley Young cut in from the left and found Javier Hernandez at the back post but the unmarked Mexican fluffed his shot.
Van Gaal had to change his plans in the eighth minute when Antonio Valencia suffered an injury and was replaced by Luke Shaw.
Liverpool then started to come into the game.
Hernandez flicked Gerrard's chip to Jordan Henderson but he could only head over.
Liverpool's biggest threat came from Sterling, who managed to find a pocket of space between Young, who had switched to the right-sided wing-back, and Phil Jones.
Sterling drew a clumsy challenge from Jones in the 14th minute and the referee pointed to the spot.
Gerrard converted to David De Gea's right and Liverpool had the lead.
The United stopper then had to stretch to deny Philippe Coutinho at his near post.
United's defence, which had looked solid all tour, appeared vulnerable to Liverpool's pace.
Sterling again broke through and was pulled down by Herrera but this time referee Mark Geiger told the winger to get to his feet.
It was much the same story after the restart
Gerrard intercepted a bad pass from Herrera and found Rickie Lambert but his shot was weak.
Sterling found the gap between Jones and Young again thanks to a cutting ball from Coutinho but Chris Smalling diverted the England international's shot wide.
Then, out of nowhere, the match swung in United's favour.
Hernandez sped down the right and cut out three defenders with a deep cross to Rooney who volleyed past Simon Mignolet.
Two minutes later, Shaw slipped a cleverly disguised pass into Mata and he beat Mignolet with a low shot which flew in off Mamadou Sakho.
Bizarre scenes followed in the 65th minute
Young's looping shot beat Mignolet but bounced back into play off the goal frame and Rooney converted while the Belgian was leant on the inside netting.
After viewing a replay, the referee chalked the goal off after it became clear the ball had bounced back into play off the stanchion, not the bar.
Liverpool made a host of substitutions but they could not alter the course of the match
United were in control now and they rounded off the win when Lingard drilled Young's pass past Mignolet with three minutes left.
Source : PA
Source: PA