Borucki was selected by the Blue Jays in the 15th round of the 2012 Major League Baseball draft, and was assigned to the Rookie-levelGulf Coast League Blue Jays. Borucki pitched six innings in the GCL, posting a 1–0 record and a 3.00 ERA before undergoing Tommy John surgery, which caused him to miss the entire 2013 season. In 2014, he pitched for the Rookie AdvancedBluefield Blue Jays and the Short Season-AVancouver Canadians, and earned a combined record of 3–2, a 2.37 ERA, and 52 strikeouts in 57 innings pitched. Borucki made only three appearances in the 2015 season, posting an 0–1 record, 3.18 ERA, and seven strikeouts in 5 innings pitched. He was assigned to the Advanced-ADunedin Blue Jays to open the 2016 minor league season. Borucki struggled with Dunedin, posting a 14.40 ERA through six starts before being reassigned to the Class ALansing Lugnuts. He fared much better with Lansing, making 20 starts and posting a 10–4 record, 2.41 ERA, and 107 strikeouts. The Blue Jays added Borucki to their 40-man roster after the season to protect him from the Rule 5 draft. Borucki was assigned to Advanced-A Dunedin to begin the 2017 minor league season. After posting a 6–5 record, 3.58 ERA, and 109 strikeouts in 98 innings, he was promoted to the Double-ANew Hampshire Fisher Cats. In seven starts with the Fisher Cats, Borucki went 2–3 with a 1.94 ERA and 42 strikeouts in 46. He made a single start with the Triple-ABuffalo Bisons before the season ended, going six scoreless innings. Borucki was named Toronto's Minor League Pitcher of the Year on October 5, 2017. He began the 2018 season with Buffalo, and through June 24, had posted a 6–5 record, 3.27 ERA, and 58 strikeouts in 77 innings pitched.
On June 26, 2018, the Blue Jays recalled Borucki from Buffalo. He made his Major League debut that night at Minute Maid Park against the Astros, taking a loss after allowing two runs on six hits over six innings with three strikeouts. Borucki became the twelfth pitcher in franchise history to throw at least six innings and allow two or fewer runs in his debut, and the first to do so since Zach Stewart in 2011. Borucki made his first start at Rogers Centre on July 2 against the Detroit Tigers, allowing two runs over seven innings with eight strikeouts, which tied him with Roy Halladay for the second-most strikeouts for a Blue Jays' pitcher in a home debut. On July 14, Borucki was optioned to the minors. He was recalled again on July 24. On August 3, Borucki earned his first major leaguewin when he threw eight innings and allowed one unearned run in Toronto's 7–2 victory against the Seattle Mariners. Borucki finished the 2018 season with a 4–6 record and a 3.87 ERA over seventeen starts, eleven of which were quality starts, the most of any American League rookie that year. On December 5, the Toronto chapter of the Baseball Writers' Association of America voted Borucki the Blue Jays' Rookie of the Year for 2018. Borucki missed the first half of the 2019 season after suffering from elbow inflammation, which was later identified as a bone spur, during spring training. He made his season debut on July 22, allowing four runs and striking out three over 4 innings in a loss to Cleveland. After making two starts, Borucki returned to the injured list on July 31. On August 9, it was announced that he underwent surgery to remove bone spurs from his left arm and would miss the remainder of the season. As the Covid 19-shortened 2020 Major League season began in late July, Borucki failed to make the Blue Jays' 30-man Opening Day roster. He was beat out by rookie Anthony Kay for the final spot in the team's starting rotation. However, he was soon activated on July 27 and added to the team's bullpen. He made his season debut that night, pitching 1 scoreless innings and earning the win against the Washington Nationals.
