Lester (5-0) did not allow a hit until Maicer Izturis, batting ninth in the order, doubled with two outs in the sixth. Izturis was the only base runner Toronto had for the entire game, which turned out to be the Blue Jays' third straight loss.
It was the third shutout and 10th complete game of Lester's eight-year career, all of it having been spent with the Red Sox. His other two shutouts came in 2008, when he posted a 16-6 record.
Boston held only a 1-0 lead until they sent eight men to the plate while scoring four runs in the seventh.
Boston had lost six of its previous seven games.
The loss went to Ramon Ortiz (0-1), a 12-year veteran recently called up from Class AAA Buffalo. Ortiz allowed just one run on four hits in five innings. Reliever Brett Cecil surrendered three hits to open the Boston sixth and set up the big inning.