Brighton, who have now won just one of their last 17 league games, made a bright start to the match and Jake Forster-Caskey did well to tee up Solly March, whose deflected effort was saved by David Forde.
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The goal was just reward for the forward, who chased down every lost cause and pressured the Brighton defence during an impressive half.
Brighton's response came through Forster-Caskey, who created space for himself 25 yards from goal. Forde reacted well to push his powerful strike away.
Bent then sent a header past the post before March missed a great opportunity for the home side after slicing wide in plenty of space 10 yards from goal.
The result leaves Brighton 22nd in the table, while Millwall move up to 17th, five points clear of the relegation zone.