The Bees forward’s glancing header with five minutes to go ensured maximum points for Uwe Rosler’s side and ended a spirited resistance from the visitors.
Donaldson was on hand to steer Sam Saunders’ superb left-wing cross just inside the far post to end his goal drought and a three-game losing run for the Bees.
Toumani Diagouraga gave the Londoners the lead just after the half hour, stroking home Donaldson’s drive which cannoned back off the post.
But Rochdale refused to roll over and almost clawed their way back into the game, their best chance coming after 60 minutes when Stephen Jordan blazed over from close range.
Brentford defender Karleigh Osborne’s downward header was cleared off the goalline by Temitope Obadeyi as the hosts went in search of the killer goal and Donaldson saw a late looping header tipped over by Dale goalkeeper David Lucas, who also denied West Brom loanee Saido Berahino’s drive from a tight angle.