This match between two play-off chasing teams looked destined to peter into a goalless draw until substitute Thorpe's late intervention.
The former Bees striker, who was let go by Andy Scott in January, came off the bench on 53 minutes and 20 minutes later broke the deadlock.
Fellow sub Ben Muirhead, who caused chaos on the right when he came on at the same time as Thorpe, crossed superbly for Thorpe to finish with a bullet header.
Earlier it was Brentford who were creating all the chances with Nathan Elder heading it across the face of goal and Glenn Poole having an effort saved in the opening exchanges.
Rene Howe headed against the post from Adam Rundle's cross from the visitors' first chance, before down the other end Poole had a diving header saved from Elder's run and cross and Alan Connell was denied when he should have done better.
Gary Jones then had a shot saved for the visitors before Elder, who lost little in the air all day, headed Ryan Dickson's cross just wide.
Brentford thought they had opened the scoring on 35 minutes when Dale keeper Tommy Lee dropped the ball in the box causing a scramble, but he quickly reclaimed it.
Poole then went close with a couple of efforts either side of half-time before Brentford's best chance so far, with Rory McArdle clearing Craig Pead's cross with Elder ready to pounce.
Poole had a free-kick saved and Connell turned and shot over as Brentford turned the screw but, just when it looked like a goalless draw was the inevitable outcome, it was that man Thorpe, who only scored three goals in his six months at Griffin Park, who opened the scoring.
As Brentford pushed for an equaliser Elder fired over an open net and Connell just failed to get to an Elder flick-on.
But gaps were appearing at the back as Brentford pushed forward, allowing Rundle to score a second on the break, again from Muirhead's, cross to give the scoreline a flattering outlook for the visitors.