Yankuba Minteh, Kaoru Mitoma and Georginio Rutter all got on the scoresheet as Albion made it three wins in a row, in all competitions – after a run of eight without a victory.
The visitors needed just four minutes to open the scoring with Mitoma breaking free and putting the ball on a plate for Minteh.
Manchester United, who were unbeaten in the previous three games, responded well and levelled through a Bruno Fernandes penalty.
It was a sloppy way to concede for Brighton as goalkeeper Bart Verbruggen’s sloppy pass was cut out by Amad Diallo and landed at the feet of Joshua Zirkzee – who was then brought down by Carlos Baleba.
It was a relatively even first-half but the visitors took complete control after the break.
Joao Pedro thought he had restored the hosts’ lead but it was ruled out for a foul by Jan Paul van Hecke on Diogo Dalot.
It didn’t matter, though, as Mitoma and Minteh combined again with the Japan star, this time, tapping in a simple finish the back post.
The in-form Rutter came on a second-half substitute for Danny Welbeck and he made an immediate impact.
The Frenchman grabbed his fourth goal in three games after Onana fumbled Yasin Ayari’s cross.
That goal effectively ended the game as a contest, as Brighton earned their second win over Manchester United this season and their sixth since May 2022.
The Red Devils have managed just two wins against the Seagulls in that time. Their last win against Brighton at Old Trafford was on February 15, 2022.
Here are our Brighton player ratings:
1. Manchester United 4-2 Brighton
Manchester United ended a run of three consecutive home defeats against Brighton. Photo: OLI SCARFF/AFP via Getty Images
2. Bart Verbruggen - 7
One of Brighton's better performers. He couldn't have done anything about any of the goals. Made some brilliant saves to keep the scoreline down after far too much sloppiness from his teammates. Vital save at 3-2. Photo: Shaun Botterill/Getty Images
3. Mats Wieffer - 5
Some promising moments going forward but had a really tough night defensively against Man United's lively attackers. Photo: Zohaib Alam - MUFC/Manchester United via Getty Images
4. Jan Paul van Hecke - 6
Intriguing battle against Benjamin Sesko, who didn't get on the scoresheet but caused plenty of problems for Van Hecke. Played a lovely pass to Watson as Brighton piled on the pressure late on. Photo: Zohaib Alam - MUFC/Manchester United via Getty Images