Brian Cook's 3-pointer with 51.5 seconds remaining gave the Wizards a 95-93 lead, and Beal, the former Florida star drafted third overall, made two free throws with 5.5 seconds left to seal the victory.
Cook and Jordan Crawford also scored 14 points apiece for Washington.
Tristan Thompson had 18 points and 10 rebounds to lead Cleveland.
Kyrie Irving, the rookie of the year last season, scored nine points on 3-of-13 shooting in 25 minutes. Irving is 11 of 46 from the field in four preseason games.
The Wizards were playing without several players, including star point guard John Wall, who is expected to be out until December with a knee injury. Emeka Okafor (stomach virus), Greg Booker (hamstring),Nene (foot) and Chris Singleton (shoulder) also missed the game.
As Kevin Love goes for the shot, Bismack Biyombo is there for the block and follows up with the signature Dikembe Mutombo salute.
The Raptors pick off Kyrie Irving's pass and feed it to Patrick Patterson, who finishes with the dunk on the other end.
LeBron James goes up for an alley-oop dunk but gets swatted by Bismack Biyombo. The refs call the foul on Biyombo, much to the chagrin of the Toronto crowd.