Memphis got the rights to Mayo, the No. 3 overall selection, in an eight-player trade just hours after the NBA draft ended.
The Grizzlies also announced the signing of Darrell Arthur, the 27th selection in the draft.
Mayo, a 6-foot-5, 200-pound guard, averaged 20.7 points, 3.3 assists and 1.6 steals a game as a freshman at USC.
Arthur, a 6-9, 225-pound forward, was originally selected by New Orleans and then traded on draft night to Portland and Houston before ending up with the Grizzlies.
He averaged 12.8 points and 6.3 rebounds as a sophomore at Kansas.