Albion's promotion surge spluttered to a halt at Field Mill.
Two second half goals by the prolific Chris Greenacre ended the Seagulls' six-match unbeaten League run and gave lowly Mansfield only their second win in twelve games.
Micky Adams' men struggled to hit top form in drizzly conditions and could have no complaints about the result.
A crowd of 2,668 endured a dour first half in which both defences were in command.
Michel Kuipers produced a fine reflex save to foil Greenacre just before the break, while Richard Carpenter had a long range shot pushed over by Mansfield keeper Bobby Mimms.
Paul Brooker brought more urgency to Albion's play as a half time replacement for Nathan Jones.
Gary Hart almost broke the deadlock in the 53rd minute with a header from a Carpenter free kick which Stuart Hicks cleared off the line.
Greenacre pounced in somewhat fortunate fashion eight minutes later.
The former Manchester City forward scored with a left foot shot on the turn when the ball fell obligingly at his feet.
Greenacre wrapped things up with a minute left, slotting home his 14th of the season as Albion pushed up in search of an equaliser.
Bobby Zamora, subject of a rejected £1.2million bid from Cardiff, had few opportunities to shine.
But he could have equalised with a 65th minute header straight at Mimms from a Kerry Mayo centre.
Albion must now bounce back from successive defeats following their F.A. Cup exit at Scunthorpe when they entertain Exeter on Friday.
Meanwhile, Adams and several of his players face their former club Brentford in the second round of the LDV Vans Trophy at Withdean in the week commencing January 8.
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