Rangers, last season's beaten play-off finalists, went through to the LDV Vans Trophy southern area semi-finals with first-half goals from Steve Palmer and Tony Thorpe.
Chris McPhee's second-half goal gave Brighton hope, but Ian Holloway's side held on comfortably.
Brighton almost took an early lead, first when Mark Yeates' corner deceived the Rangers defence and was cleared off the line by Martin Rowlands, and then when Daniel Harding's left-foot shot struck the post.
But Rangers drew first blood when Kevin Gallen swung in a corner and Palmer's header was tipped on to the bar by Ben Roberts before crossing the line.
Worse followed for Albion when Thorpe pounced on a error by youngster Adam El-Abd and lobbed Roberts to make it two.
Brighton improved after the break and Zeshan Rehman and Yeates both sent shots wide of the target.
And the Seagulls got the goal they deserved when McPhee's deflected shot looped over keeper Nick Culkin and into the corner of the net, but they couldn't find an equaliser.