Marvin Williams' second-half equaliser was not enough to stop Millwall falling to bottom spot in the Championship.
Plymouth went ahead from their first meaningful move, Vincent Pericard heading home Paul Connolly's corner.
Alan Dunne almost brought the hosts level when his header was saved, before 18-year-old Williams did so with a brave header from close range.
Marvin Elliott came close to a winner for the hosts, but the game drifted to a draw to leave Millwall on the brink.
Millwall caretaker manager David Tuttle:
"It's finished now, and we are relegated, the task now is to prepare for next season and League One.
"It's sad a club like Millwall will be fighting in League One, but we need to be prepared as we can't afford to drop through another division.
"Everyone needs to look themselves in the eye but I won't be the manager."
Plymouth boss Tony Pulis:
"It's nice for Plymouth to come to a place like this and not be satisfied with a point.
"I don't want to do my own lads an injustice but Millwall had it all to play for and they showed great grit and determination today.
"We confirmed our Championship status a while ago and are on the up."
Millwall: Doyle, Lawrence, Phillips, Whitbread, Elliott, Dunne (Cogan 76), Livermore, Williams, Craig, Asaba (Powel 42), May.
Subs Not Used: Marshall, Vincent, Braniff.
Goals: Williams 31.
Plymouth: Larrieu, Connolly, Doumbe, Aljofree, Hodges, Norris, Nalis, Wotton, Capaldi (Buzsaky 60), Clarke (Evans 64), Pericard (Pulis 79).
Subs Not Used: McCormick, Chadwick.
Booked: Wotton, Pulis.
Goals: Pericard 10.
Ref: P Walton (Northamptonshire).
The BBC is not responsible for the content of external internet sites