Brighton's inspirational skipper Dean Hammond fired his side ahead just before the break and second-half goals from Bas Savage, Joe Gatting and Dean Cox sealed Orient's fate despite a consolation effort from substitute Jason Demetriou.
With so much at stake both sides were afraid to give anything away in the opening stages as the relegation-threatened duo cancelled each other out.
Phil Mulryne, playing his first game since October 2005 for injury-hit Orient, set up the first chance on 20 minutes when his astute chip found James Walker, but he spun and fired over the bar from eight yards.
As the O's got into their stride Gary Alexander raced through but from a tight angle fired into the side netting.
But it was Brighton who made the all-important breakthrough on 39 minutes.
Orient failed to clear their lines and when Savage crossed from the right the ball fell neatly at the far post for Hammond to fire into the net.
The dangerous Shane Tudor almost levelled with Orient's first shot on target on 44 minutes, but Brighton should have stretched their advantage on the stroke of half-time but Gatting failed to find a finish from close range.
After the interval Orient almost levelled when Wayne Corden fired over the bar but then the visitors took a vice-like grip.
On 57 minutes Cox raced down the right and crossed for Savage to hammer home his side's second from close range.
Ten minutes later the Seagulls added a third when Cox caused the damage with another cross from the right, Savage headed the ball back into the danger area and Gatting swept it home.
Orient reduced the deficit three minutes from time when Tudor played a quick free-kick in from the right and Demetriou ghosted in to head home his first-ever goal for the club.
But there was to be no dream comeback for the home side and Brighton rubbed salt into their wounds with a fourth goal a minute from time.
Substitute Nathan Elder flicked the ball on and Cox raced clear of the defence to fire home to complete a marvellous Seagulls victory.