Neither side were able to test their opponents' goalkeeper in the first half, with chances few and far between in the opening 45 minutes.
The Lions defended their lead well until the 75th minute when Alan Dunne picked up his second yellow card of the match, leaving Millwall a man short for the final 15 minutes.
Kazenga LuaLua and Ulloa both had chances for the home side, but struggled to find the target as Brighton pushed for an equaliser.
However, Ulloa levelled the scores in the 89th minute, lashing the ball in from a LuaLua delivery.
Will Buckley tried to force in a winner for the Seagulls in stoppage time, but Millwall were able to clear the danger.