Leonardo Ulloa struck a late, late leveller to deny 10-man Millwall their first Championship win of the campaign in a pulsating game at the Amex Stadium.
Martyn Woolford's second goal of the season on 50 minutes had put The Lions bang on course for that elusive first League victory, but the dismissal of defender Alan Dunne 15 minutes from time for his second booking left Steve Lomas' tiring charges a man down at a critical point in proceedings.
Having made a midweek trek to Nottingham Forest and endured extra-time, Millwall dug in at the Amex Stadium but were finally undone as the game approached the final minute when Ulloa knocked home from close range.
It left Lions boss Lomas wondering what might have been after seeing his men come within touching distance of the three points that their performance arguably deserved.
Millwall settled quickly into their stride and it took a smart defensive clearance by Inigo Calderon to turn Dunne's dangerous cross behind for the first corner of the game, from which Woolford's header was saved by Tomasz Kuszczak in the home goal.
The Lions, showing six changes to the team that faced Forest on Wednesday, created a couple of delightful moves in that opening quarter of an hour down the right with Dunne and Henry combining well.
However it was Brighton midfielder Kemy Agustien who went closest with a thumping 18-yard drive that flew narrowly over the bar.
Seagulls defender Stephen Ward, on loan from Wolves, picked out Ulloa after 23 minutes, but he was unable to wriggle free of Mark Beevers and the Millwall defence again held firm.
At the other end The Lions, and Scott McDonald in particular, produced some delightful, intricate moves but without finding that decisive pass or touch in front of goal during the opening 45 minutes.
Brighton brought on Andrea Orlandi for Agustien at the start of the second half but it was Millwall who came flying out of the traps and a fine lay-off by McDonald played in Henry, whose right-foot drive flashed narrowly over secods after the restart.
McDonald, who put in a fantastic shift, saw a mazy run on 47 minutes finally come to an end - but it took three home defenders to bring him to a halt.
However Millwall's bright play finally got its reward after 50 minutes when great build-up play by McDonald found Richard Chaplow, who in turn slipped the ball through to Woolford. His angled low finish was as clinical as it was cool.
Just six minutes later right-back Dunne was to pick up the first of his yellow cards, and how costly his time-wasting antics at a throw-in proved to be as he went into referee Bobby Madley's book.
Brighton brought on Kazenga LuaLua just before the hour mark and with that Brighton looked to utilise the scintillating pace of their wide man at every opportunity.
Twice the sub went close with shots from distance that were blocked whilst Lions left-back Shane Lowry made a stunning block from Rohan Ince, throwing his body fully to divert the danger.
Whereas The Lions' recent defensive displays had not been as resilient or assured as usual, Lomas' team could not be faulted for their rearguard action on this occasion.
Chaplow took the full weight of a LuaLua shot with Ince and Orlandi then put off sufficiently to fire wide from central positions outside the box.
Millwall went close to doubling their tally with 18 minutes remaining on the counter-attack when McDonald and Woolford again combined, the latter's low shot from a near identical position to his goal on this occasion held by keeper Kuszczak at his near post.
Brighton had the ball in the back of the net moments later, but the celebrations were cut short when Ulloa was adjudged to have pushed to gain an advantage as he headed home.
Those groans fromthe home fans turned to grins on 76 minutes though when Dunne, already booked 20 minutes earlier, was shown a second yellow card for a foul on Orlandi.
Lomas responded by bringing on Liam Trotter for Chaplow for his first League action of the season with fellow sub Jimmy Abdou dropping back to fill the defensive gap left by Dunne.
Brighton kept up the pressure, however, and it took a brave save by David Forde at the feet of Will Buckley to deny the Brighton man.
Forde, back in the first team after being sidelined since the opening day of the season, then produced a wonderful save to tip over Orlandi's piledriver.
Unfortunately there was little he could do when, in the dying seconds of normal time, LuaLua burst past Abdou and despatched a perfect cross that Ulloa swept home at the far post to level the game up.
With Brighton relentless in their bid for a second goal it took a brilliant goalline clearance from Lowry to prevent Buckley from grabbing what would have been a real sickener of a winner in the 93rd-minute.
There was still time for Orlandi's left-foot curler to drop inches the wrong side of the upright right at the death, but in the end Millwall held firm and were good value for their second away point in a week.
Brighton & HA: Kuszczak, Calderon, Greer, Upson, Ward, Ince (72mins), Crofts, Agustien (Orlandi, 46mins), Buckley, Lopez (LuaLua, 57mins), Ulloa. Subs not used: Ankergren, El-Abd, Andrews, Forster-Caskey, March.
Bookings: Ince (13mins), Calderon (45+3mins)Scorer:
Millwall: Forde, Dunne, Lowry, Bailey, Shittu, Beevers, Woolford, Derry, Chaplow (Trotter, 78mns), McDonald (Easter, 84mins), Henry (Abdou 62mns). Subs not used: Bywater, Robinson, Keogh, Martin.
Booking: McDonald (30mins)
Sending off: Dunne (76mins)
Scorer: Woolford (50mins)
Att: 26,804 (away 1,778)