"It was just one of those days where you keep getting opportunities to throw the ball," said Mastroianni, who is batting .206 in 29 games with New Hampshire after hitting .325 in 61 contests at Class A Advanced Dunedin.
"Obviously, you want to do well offensively. It's hard not to take it out there sometimes. It's something we talked about a lot with the coaching staff. I can play defense and run. That's one of the main reasons I'm here. Center field is an important position no matter what level you're at."
Mastroianni, Toronto's 16th-round pick in the 2007 Draft, began flashing his defensive prowess in the opening frame.
Lars Anderson's two-out double plated Jorge Jimenez with the game's first run, but Mastroianni prevented Portland from jumping out to a 2-0 lead when he fired in to shortstop Luis Sanchez, whose relay to catcher Brian Jeroloman nailed John Otness at the plate.
After the Fisher Cats jumped in front, 2-1, in the top of the second, Mastroianni didn't need any help preventing the Sea Dogs from tying it up in the bottom of the frame. Jon Still walked and moved to second on a balk before trying to score on Matt Sheely's two-out single to center.
Mastroianni came up throwing and fired a perfect strike to Jeroloman, who did a nice job of blocking the plate and applying the tag to end the inning.
"I couldn't have done it without my teammates," said Mastroianni, who went 1-for-5 with a run scored and picked up his 14th stolen base for New Hampshire after swiping 32 with Dunedin.
"We had a couple of relays. They did a great job of throwing guys out at the plate. You need help when you make those types of throws. They got me two more assists right there."
The speedy Mount Kisco, N.Y., native singled, stole second and came home on Scott Campbell's double in the fifth to give the Fisher Cats a 3-1 lead before getting an unexpected assist in the bottom half.
Ryan Khoury led off with a double and scored on a single to center by Sheely as Mastroianni's throw home was a little late. Sheely, trying to move up on the play, scampered toward third when Jeroloman's throw sailed into center.
Again, Mastroianni was poised to make a play. He raced in and collected the ball before cutting down Sheely at third for his third assist.
Still's two-run single in the sixth gave the Sea Dogs a 4-3 lead, but Mastrioanni wasn't done. Yamaico Navarro followed with a double to center, but Mastroianni recovered the ball quickly, turned and fired into second baseman Jonathan Diaz, who nailed Still at the plate to make New Hampshire history.
"When the fourth one came about, we all kind of knew what happened, but we were more concerned with the game," Mastroianni said. "It was cool, but it wasn't the main focus at the time."
The record-setting performance came at a good time for the Fisher Cats' leadoff man, who is batting .162 (6-for-37) over his last 10 games.
"The [offensive] numbers aren't there, but there's an adjustment anytime you jump a level," Mastroianni said. "It's a pretty big jump. I'm seeing the ball well and just getting used to the new pitchers. It's going to take some time. I feel good at the plate. Honesty, I feel like it's just a slump right now."
Mastroianni had nine outfield assists at Dunedin and has added six to that total with the Fisher Cats.
"Even when you're 0-for-5 in a game, you have to go out there and maintain your focus," he said. "You can still help the team."