It was a confidence booster for the Bees ahead of Monday night's Johnson's Paint Trophy Southern Final clash between the two sides.
Forster's 94th minute equaliser came when he hammered home after a goalmouth melee to earn the Londoners a deserved point after John O'Flynn had given the Grecians a second minute lead.
He capitalised on a slip by Brentford left-back Craig Woodman to run on and fire an angled drive into the bottom corner.
The first half was all Exeter. Liam Sercombe fired over the angle and Troy Archibald-Henville headed wide when unmarked.
The Bees only effort before the break came when Steve Tully saved Charlie MacDonald's low drive only for Marc Laird to sidefoot against the upright.
It was a different story after the break, Marcus Bean finding the side netting for the home side as they laid siege to the visitors' goal.
Keeper Tully was the hero saving a 20 yard piledriver from Myles Weston before tipping MacDonald's smart turn and shot on to the bar
When MacDonald side-footed wide from close range late on it looked like a goal would never come - until Forster's late, late strike.
Brentford 1-1 Exeter: Match Report - view commentary, squad, and statictics of the game as it happened.