The Blue Jays, on the outskirts of the AL wild-card race, announced the deal Sunday night. Mayberry would be eligible for the postseason if Toronto reaches the playoffs.
The 30-year-old Mayberry hit .213 with six home runs and 21 RBIs in 63 games for the Phillies this season. He'd been on the disabled list since July 21 because of inflammation in his left wrist, and was expected to be activated Monday.
Mayberry arrived in the Phillies' clubhouse before Sunday's game in New York, and had been scheduled to fly with the team to Atlanta after its 6-5 loss to the Mets.
In six seasons with the Phillies, Mayberry hit .242 with 52 homers and 169 RBIs.
The 22-year-old Pierre batted a combined .260 with eight homers and 43 RBIs at Class A and Double-A.