McGregor was taken in the 15th round of the 2008 MLB draft, a couple picks after outfielder Joey Butler, by the St. Louis Cardinals. He was 4–0 with a 1.45 ERA and five walks allowed in 37.1 innings his first professional season and followed that with a 7–11 record and a 5.56 ERA in 34 games (17 starts) for the Quad Cities River Bandits in 2009. In 115 innings, he allowed 26 walks and had the eighth-best BB/9 IP ratio in the Midwest League among pitchers with 100 or more innings pitched.Midwest League leaders In 2010, he was 10-8 with a 3.14 ERA in 31 games (20 starts) split between two teams; in 137.2 innings, he allowed 22 walks. Baseball America ranked him as having the best control in the Cardinals system. After missing 2011 to Tommy John surgery, McGregor returned in 2012 to go 5–2 with a 6.31 ERA in 14 games (13 starts). He reached Triple-A for the first time in 2013, going 6–10 with a 4.83 ERA in 18 games (17 starts) for the Memphis Redbirds; He also pitched for the Springfield Cardinals that year and went 10–11 with a 4.06 ERA as a whole. He began 2014 with Memphis, but was released in June.MLBTradeRumors.com The Nationals signed him and he played for their Double-A team, the Harrisburg Senators, and their Triple-A club, the Chiefs. Between the three teams, McGregor was 4–9 with a 5.56 ERA. In 2015, he was 6–6 with a 4.04 ERA in 27 games (15 starts) for Syracuse.BR Minors page McGregor was released on March 16, 2016.
