Martyn Woolford drove the Lions ahead shortly after the break and the visitors had Alan Dunne sent off for two bookable offences before Ulloa struck a minute from time.
Millwall settled quickly and Brighton's Spanish defender Inigo Calderon put a Dunne cross behind for the first corner of the match before a goal-bound header from Woolford was saved by goalkeeper Tomasz Kuszczak.
Brighton's Dutch midfielder Kemy Agustien drove wide from the edge of the area in the 16th minute after the ball was squared to him by Rohan Ince.
The Seagulls went into the game with Argentinian Ulloa as their only fit striker and he was well kept in check by Lions skipper Danny Shittu in the early exchanges.
On-loan Wolves defender Stephen Ward crossed invitingly for Ulloa midway through the first half but the big forward was unable to wriggle free.
Ince later had a low shot blocked following a surging run down the middle from Wales international Andrew Crofts.
Millwall did a good job of stifling Brighton in the first half and Shittu was there to clear a dangerous low cross by Will Buckley shortly before the break.
The Lions had David Forde back in goal following a knee injury and the Irishman was not tested in the opening period as Brighton struggled to break down a stubborn visiting defence.
Midfielder James Henry burst through for Millwall moments into the second half but put his shot from the edge of the box over.
Millwall broke the deadlock after 51 minutes through Woolford who calmly slotted home from close in after being set up by Richard Chaplow.
Brighton boss Oscar Garcia, having introduced Andrea Orlandi for Agustien at the break, threw on Kazenga LuaLua in place of David Lopez, and the winger twice had shots deflected wide.
Former England defender Matt Upson later shot wide from an Orlandi corner before LuaLua drove over as the Seagulls looked more dangerous.
Crofts came close to netting his third goal in three games on 67 minutes when his curling left-footed effort flew inches wide.
Woolford had a low shot saved by Forde at the other end following a pass from Scott McDonald before Ulloa had the ball in the net from a free kick from Orlandi, but the effort was ruled out for pushing.
Millwall were reduced to ten men 14 minutes from time when Dunne received his second yellow card for bringing down Orlandi.
Orlandi had a superb shot from the edge of the box tipped over by Forde before Ulloa rescued a point for Brighton at the far post from a cross by LuaLua.