Donaldson broke the deadlock midway through the first half, latching on to Adam Forshaw's through-ball to coolly slot beneath goalkeeper Paul Jones.
A defensive blunder from Bees right-back Shaleum Logan almost handed Crawley a point when his wayward back pass dropped for former Brentford midfielder Nicky Adams, who bundled home from close range.
But Donaldson had the final say after 76 minutes when he raced on to a lofted pass from Tom Adeyemi to lift the ball over goalkeeper Jones from the edge of the box.
Brentford should have wrapped things up far earlier, but Jones palmed Logan's 30-yard pile-driver clear and pulled off a reflex save to tip Donaldson's close-range header wide.
The only black mark on a good day for Brentford came when striker Farid El Alagui was stretchered off with a serious leg injury.