The visitors were rocked by an injury in the warm-up to first-choice goalkeeper Emi Martínez, with Marco Bizot coming in for just his third PL start.
And the 34-year-old failed his first major test when he flapped at a ninth-minute corner, allowing Jan Paul van Hecke to nod home only his second Brighton goal after some pinball on the line.
It was the fifth PL goal Villa had conceded in the first 15 minutes this season, with none scored.
But far from rally immediately after that, they went 2-0 down just before the half-hour mark.
Jack Hinshelwood was sent racing through on goal and his square pass cannoned off Pau Torres and past Bizot for an unfortunate own-goal.
Brighton had now scored at least twice in six successive home games, but the momentum was about to shift dramatically.
Villa had been firmly second-best, but pulled a goal back eight minutes before half time, as Ollie Watkins bravely slid home in a crowded six-yard box to convert Ian Maatsen’s low cross.
Ezri Konsa then headed against the post deep in stoppage time - and there was still time for a fourth goal of a breathless half.
Watkins raced on to a hopeful long ball and fired low past Bart Verbruggen for his ninth goal in 10 league H2Hs.
Those two goals put Villa in the driving seat, and they completed the comeback on the hour mark.
Matty Cash swung a deep corner to the far post and Amadou Onana headed home from close range right in front of the visiting fans, sparking bedlam in the away end.
Lewis Dunk then almost turned Evann Guessand’s cross into his own net on his 500th Brighton appearance, but Verbruggen’s reaction save rescued the Seagulls skipper.
Villa continued to press, and their fourth goal arrived 12 minutes from time, when Donyell Malen – who had come on as a substitute just 25 seconds earlier – prodded home after Verbruggen parried Guessand’s header.
However, the contest was not over, as Van Hecke curled home from the edge of the penalty area seven minutes from time to set up a grandstand finish.
Bizot brilliantly saved Danny Welbeck’s header in stoppage time but that was the closest Brighton came, as they suffered a first home league defeat of the season.
Flashscore Man of the Match: Ollie Watkins (Aston Villa)
