Glasnow signed with the Pirates for a $600,000 signing bonus. He made his professional debut in 2012 for the Gulf Coast Pirates of the Rookie-levelGulf Coast League in 2012 where he went 0–3 with a 2.10 ERA in 11 games, and also started one game for the State College Spikes of the Class A-Short SeasonNew York–Penn League. In 2013, he played for the West Virginia Power of the Class ASouth Atlantic League. He started 24 games and finished the season with 9–3 record, a 2.18 ERA and 164 strikeouts in 111.1 innings. His 164 strikeouts were the most strikeouts in a single season in Power franchise history, passing Will Inman's 134 in 2006. Glasnow played for the Bradenton Marauders of the Class A-AdvancedFlorida State League in 2014. In 23 starts for Bradenton, he compiled a 12–5 record and 1.74 ERA. After beginning the 2015 season with the Altoona Curve of the Class AAEastern League, he sprained his ankle on May 6, and made his return on June 19 with the West Virginia Black Bears of the New York–Penn League. After two starts with West Virginia, Glasnow returned to Altoona. In late July, the Pirates promoted Glasnow to the Indianapolis Indians of the Class AAAInternational League. The Pirates considered promoting Glasnow to the major leagues in 2015, but decided against it. In 22 starts between the three clubs, Glasnow was 7–5 with a 2.39 ERA and 136 strikeouts in 109.1 innings. After the 2015 season, the Pirates added Glasnow to their 40-man roster. Glasnow began the 2016 season with Indianapolis and had a 1.87 ERA and 133 strikeouts in innings pitched across 20 games started. The Pirates promoted Glasnow to make his major league debut on July 7. He pitched innings in his major league debut, giving up four runs on three hits and two walks. In his second start, he left the game after three innings with a shoulder injury. He returned to the active roster on July 23 as a relief pitcher. He did not start another game until September 25. In 23.1 innings pitched for Pittsburgh, Glasnow was 0–2 with a 4.24 ERA. Glasnow began 2017 in Pittsburgh's starting rotation. In his first start of 2017, he struggled with his command, giving up five runs on four hits and five walks in innings pitched. Glasnow was optioned to Indianapolis in June after compiling a 7.45 ERA and 1.91 WHIP over 12 starts. He spent the remainder of the season with Indianapolis, where he was 9–2 with a 1.93 ERA over 15 starts, before he returned to Pittsburgh during September call-ups. In 15 games for the Pirates, he compiled a 2–7 record, a 7.69 ERA, and a 2.012 WHIP. During spring training in 2018, the Pirates decided that Glasnow would start the 2018 season as a relief pitcher.
Tampa Bay Rays
On July 31, 2018, Glasnow was traded to the Tampa Bay Rays, along with Austin Meadows and a player to be named later, later revealed to be Shane Baz, for Chris Archer. He was immediately inserted into their starting rotation. In his 11 starts with Tampa Bay, Glasnow posted an earned run average of 4.20, recording 64 strikeouts in innings. After starting the 2019 season 5–0 with a 1.75 earned run average, Glasnow was named American League Pitcher of The Month for April. After straining his arm against the Yankees on May 10, Glasnow went to the 10 Day IL. He went to the 60 day IL on May 26. He returned in September and made four starts, not finishing past the fifth inning in any of them. Glasnow finished the 2019 season with a 6–1 record and a 1.78 ERA in innings.
