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:
9. Ferdi Kadıoğlu - 7
The versatile Turkish international was deployed in attacking role, with a lack of natural wingers available. Played on both flanks and made a good account of himself. Forced Lammens into a great save at full stretch at 3-1. Booked for foul on Mbeumo at 3-2. Photo: Zohaib Alam - MUFC/Manchester United via Getty Images
10. Georginio Rutter - 5
Squandered huge chance on stroke of half-time. He fluffed his lines from point blank range. Pleaded in vain for a foul in build-up to Mbeumo's first goal. Not his best performance but you can't fault his efforts. Photo: Shaun Botterill/Getty Images
11. Yankuba Minteh - 6
Brighton's biggest threat but very wasteful. Nearly scored after Messi-like run in the first-half. To his credit, he kept getting in the right positions. Looked to be through on goal when he was brought down by Dorgu. Welbeck scored the free-kick. Photo: Carl Recine/Getty Images
12. Danny Welbeck - 8
Almost guaranteed to score against Man United. Made it five goals for the season with a fantastic free-kick. Nearly sparked a crazy comeback. He was actually playing well even when Brighton were three goals down, with some lovely touches. What a player and what form he's in right now. Are you watching Thomas Tuchel? Photo: Carl Recine/Getty Images