Billy Dearden's depleted strugglers arrived on the south coast one place off of the foot of the Football League without an away win all season and with only 16 fit senior players.
A place in the fourth round draw for the first time in 15 years beckoned for League One play-off contenders Brighton, but the visitors stunned them with an inspired display.
The Stags, minus injured top scorer Michael Boulding, were the better side throughout and Brighton could have no complaints about their humiliating exit.
A surprise was on the cards from the tenth minute when Mansfield went ahead.
Simon Brown's pass caught Brighton full-back Joel Lynch napping.
Matt Hamshaw ran in behind him to find the net with a shot from an acute angle which found its way in via the body of goalkeeper Michels Kuipers.
Normal service looked like being resumed midway through the first half when equally sloppy defending cost Mansfield their lead.
Alex Revell was left unmarked from a Dean Cox free-kick and the in-form Brighton striker headed his fifth goal in three games.
The home fans sat back expecting the floodgates to open in much the same way as they did when Brighton mauled Mansfield 6-0 in their first competitive fixture at the Withdean Stadium nine years ago.
Instead the underdogs, belying odds of 8-1 against a victory, continued to look the better side and restored their lead in first-half stoppage time.
The Brighton back four were caught sleeping again as Johnny Mullins crossed low for Boulding's deputy Ian Holmes to side-foot into the roof of the net from close range.
Brighton boss Dean Wilkins, missing injured captain Dean Hammond and unwell defender Andrew Whing, rang the changes in the second half but it made no difference.
Kuipers prevented further goals for the visitors with fine saves from Brown and Stephen Dawson.
The closest Brighton came to an equaliser was a downward header by substitute Nathan Elder from Revell's cross which was grabbed by Mansfield's rarely troubled veteran keeper Carl Muggleton.