Selected 11th overall in the 2010 Draft, McGuire scattered three hits and walked two while striking out four over 5 2/3 innings in the Class A Advanced Dunedin Blue Jays' 4-0 victory over the Lakeland Flying Tigers.
"It's always nice to get that first one out of the way," McGuire said.
"I threw my fastball to both sides of the plate, threw my cutter to lefties and didn't show my offspeed stuff unless I had too. Everything worked well."
The Toronto prospect allowed a leadoff double to Gustavo Nunez in the first inning, but he retired the next eight batters in a row before Nunez doubled again with two outs in the third.
As in the first frame, McGuire stranded the runner in scoring position, this time striking out Daniel Fields to keep the 1-0 lead intact.
The 21-year-old worked around a one-out walk in the fourth, retired the side in order in the fifth and took the mound in the sixth staked to a 4-0 advantage.
Nunez singled and Daniel Fields drew a walk to start the inning, but McGuire kept a run from coming home by striking out Jordan Lennerton and getting Wade Gaynor to fly out to center field.
"With the exception of Nunez, I thought I pitched every hitter well," McGuire said. "I was throwing the ball well, but I just couldn't get him out.
"It was totally my fault for the runners getting on base, but it was important for me to get those next guys out."
McGuire was replaced with two outs by Aaron Loup, who struck out Tony Plagman to close the book on the right-hander's night and put him in line for the victory.
Loup pitched a scoreless seventh and Frank Gailey shut down the Flying Tigers over the final two frames to complete the shutout.
McGuire pitched four shutout innings in his professional debut against Clearwater on April 7 and then surrendered three runs on five hits and two walks while fanning five Wednesday against the Flying Tigers.
"The biggest difference tonight was that I kept the ball down and stayed out of trouble," McGuire said.
"I got ahead in the count which I didn't do last time, and I put the ball where I wanted it."
Following Monday's performance, McGuire lowered his ERA to 1.88, and he has 14 strikeouts in 14 1/3 innings.
As a pitcher at Georgia Tech, McGuire was 27-7 in 48 games, including 44 starts.
His best year in college came as a sophomore in 2009, when he went 11-2 and struck out 118 batters in 100 1/3 innings en route to being named the ACC's best pitcher.
Despite the success as the ace of the Yellow Jackets, McGuire doesn't feel too much pressure with being a top Draft pick.
"Right now I'm a Dunedin Blue Jay and I'm just like any one of the guys," he said. "It doesn't matter where you got drafted. We are all trying to get better and help each other get better and move through the ranks."