Richard Carpenter had given Brighton the lead before an own goal put United back on level terms.
United should have snatched the lead in the sixth minute after Andy Gray got the better of Paul Reid to cross from the left flank. Andy Liddlell sliced the ball wide of the target as he charged in and Leigh Bromby stuck out a foot to hammer it into the sidenetting from close range.
Brighton, unbeaten in four, went close 12 minutes later. Carpenter's corner was met by midfielder Charlie Oatway whose header was met by Gary Hart who glanced the ball across the face of the goal from six yards.
Phil Jagielka headed wide before Michael Tonge almost found a way through from Liddell's corner, the United midfielder's low drive from the edge of the area flashing a fraction wide of the near post.
Brighton, totally outplayed, caught United on the break a minute before the interval, Carpenter threading the ball to Reid who dragged his shot wide from the 12 yards.
They snatched the lead in injury time, Hart riding Jagielka's tackle to pick out Knight, the little striker unselfishly passed inside to Carpenter who drilled the ball into the left-hand corner from 15 yards.
Tonge's darting run took him past two defenders before he produced a fierce strike driving the ball wide of the target from the tightest of angles.
United were back level in the 60th minute, Adam Virgo deflecting a through ball into the area as the unfortunate Kerry Mayo lifted it over stranded Michel Kuipers and into the net.
Reid carved out Brighton's winner stabbing the ball to Knight, who steered it past Paddy Kenny for his fourth of the season.