Mastroianni, 26, was designated for assignment by Toronto last week to make room on its roster for reliever Francisco Cordero.
He made his Major League debut last season, going 0-for-3 against the Royals on Aug. 24. It was his only big league appearance, as he spent most of the year split between Double-A New Hampshire and Triple-A Las Vegas.
He combined to hit .268 with a .353 on-base percentage and .377 slugging percentage, with three homers, nine triples, 26 doubles and 34 stolen bases in 123 games in the Minors last year.
His best season came in 2010, when he led the Double-A Eastern League in stolen bases with 46, tied for the league lead in hits with 156 and was second in runs with 104 and walks with 77.
Mastroianni, who was selected by the Blue Jays in the 16th round of 2007 First-Year Player Draft, has a career .279/.370/.372 line in four Minor League seasons. He's served mostly as a center fielder in the Minors with 377 games played there, while he's played 78 games in left field and four games in right. He also played 63 games at second base in '07.