Mastroianni, who is hitting .262 with a .347 on-base percentage, has 11 stolen bases in 40 games, including 21 starts.
"I just like it when he takes first and then he can take second and third," manager Ron Gardenhire said. "First, you have to get on. He's very aggressive and confident. He studies the pitchers and gets really into it. And he's not afraid to talk about it."
Mastroianni, who didn't start on Sunday with Ben Revere back in right field, was at it again on Tuesday, when he stole second and third on back-to-back pitches against former Twins left-hander Francisco Liriano.
"I knew as soon as I got on, I was going to steal second and third," Mastroianni said. "I've seen Frankie pitch enough. I knew if I got on, I could run on him. We all knew that. When you play with a guy enough, you notice these things."