Dean Cox was the star of the show. Cox, the smallest player on the pitch, tormented the visitors and capped an outstanding display with a stunning goal.
Brighton were fast out of the blocks and opened the scoring in the 12th minute.
Skipper Dean Hammond headed a corner from Cox back across the face of the Millwall penalty area. The ball was not cleared and, as Hammond closed in, he was upended by Gary Alexander. Hammond picked himself up to score from the spot, his second penalty of the season and third goal in as many home games.
Cox doubled Brighton's advantage in spectacular fashion seven minutes later. The ball fell to him 25 yards out and, spotting keeper Chris Day off his line, he looped in an exquisite effort.
Brighton could have been even further ahead by the break. Cox was just off-target with a long-range header and Bas Savage put Nicky Forster clean through, but he delayed and Millwall defender Zak Whitbread recovered to shut him out.
Joel Lynch hit a post from close range from a corner for Brighton early in the second half and Cox almost repeated his goal, but Day was equal to it this time and palmed his shot over.
Millwall briefly threatened a comeback in the second-half. Dave Brammer had a 35-yard drive pushed over by Brighton keeper Michels Kuipers and substitute Jay Simpson, on loan from Arsenal, skewed wide when well placed.
But they were killed off 12 minutes from time, as substitute David Martot converted a cross from Forster at the far post with a right-foot shot.
Millwall's misery was completed by a red card in stoppage time for Whitbread. He had already been booked when he brought down Forster.