Millwall 0 Brighton & Hove Albion 1
The hosts missed a hatful of chances in an opening 45 minutes littered with goalmouth action, but Adam Virgo's header took the three points back to the South Coast.
The Brighton backline withstood an early onslaught by the rampant play-off contending Lions.
Record goal-scorer Neil Harris had the best of the chances and should have scored on eight minutes when he blazed over from close-range, after keeper Michels Kuipers parried Adam Bolder's fierce drive back into the six-yard box.
Debutant Vincent Pericard was played in neatly by Bolder three minutes later but Kuipers again thwarted the attack.
A glancing Harris header from Scott Barron's pinpoint cross forced the save of the day from the Brighton keeper, who tipped round his post, with the ball destined for the top corner.
The Seagulls almost stole the lead seconds before the interval but Chris Birchall somehow blazed over in front of an open goal.
The second half was a much slower pace and only threatened to break back into life when Pericard lashed a low shot just wide of the mark on 62 minutes.
The match-winning goal for the visitors arrived on 73 minutes. Birchall's inswinging corner from the left was met powerfully by Virgo, who steered his header past helpless Lions keeper David Forde.
Bolder danced through midfield before teasing a low shot across Kuipers and wide as the minutes ticked away for the home side.