The sides were level at 2-9 each with 12 minutes remaining when Westmeath centre-forward Brendan Murtagh was sent off for a second yellow card offence.
Just three minutes later Westmeath were reduced to 13 players when half-forward Brian Smyth was shown a straight red after an altercation with Antrim’s Kevin Molloy.
Antrim took advantage to finish strongly, with two points from Neil McManus, and one each from James Black and Karl Stewart securing the win in the closing minutes.
The first-half of a very low-key game had ended level, 2-2 to 1-5, with Antrim full-forward Conor Carson notching two goals and Westmeath’s Alan Devine finding the net from close range just before the break.
The Ulster side always looked the more likely winners and Dinny Cahill’s side would have won by a greater margin except for some poor shooting.
However, with Karl Stewart, Shane McNaughton and Neil McManus in top form, the Antrim men moved three points ahead midway through the second-half.
Westmeath rallied and a goal and two points from corner-forward Dan Carty propelled them back in front, 2-9 to 2-7 with 14 minutes left.
Antrim drew level with points by McNaughton and McManus and when the two Westmeath players were sent off, the Ulster side scored four points without reply to run out deserving winners.
Scorers for Antrim: N. McManus (0-7, 0-4 from frees), C. Carson (2-0), K. Stewart (0-3), S. McNaughton (0-2), J. Black (0-1).
Westmeath: D. Carty (1-4, 0-4 from frees), A. Mitchell (0-4, 0-3 from frees, 0-1 from a ‘65), A. Devine (1-0), Anthony Price (0-1).
ANTRIM: S. Hawes; K. McGourty, S. Shannon, K. Molloy; A. Graffin, S. McCrory, J. McCouaig; M. Herron, S. McNaughton; N. McManus, K. Stewart, M. Armstrong; D. Hamill, C. Carson, B. Fall. Subs: J. Black for M. Armstrong (h-t), C. McGuinness for B. Fall (49), P. Shiels for D. Hamill (49).
WESTMEATH: P. Burke; E. Price, D. McCormack, A. McGrath; J. Shaw, A. Mitchell, P. Gilsenan; J. Clarke, N. Flanagan; C. Curley, B. Murtagh, N. Dowdall; A. Devine, B. Smyth, D. Carty. Subs: Anthony Price for C. Curley (half-time), C. Scally for P. Burke (55), C. Mackey for N. Dowdall (58).
Referee: Declan O’Driscoll (Limerick).