Giolito attended Harvard-Westlake School in Studio City, Los Angeles, California. He was part of a Harvard-Westlake baseball team that included Max Fried and Jack Flaherty, with Ethan Katz as their pitching coach. In March 2012, his senior year, Giolito sprained the ulnar collateral ligament in his right elbow, sidelining him for the rest of his high school season. Giolito had verbally committed to attend the University of California, Los Angeles and play for the UCLA Bruins baseball team. Although some thought he was good enough to be selected with the first overall choice of the 2012 MLB draft, concerns about his elbow caused him to fall to the 16th overall selection, where he was picked by the Washington Nationals.
On June 28, 2016, the Nationals added Giolito to their active roster and he made his major league debut that night against the New York Mets. In his debut, he pitched four shutout innings, allowing one hit and two walks, while getting one strikeout.
Chicago White Sox
On December 7, 2016, the Nationals traded Giolito, Reynaldo López, and Dane Dunning to the Chicago White Sox for Adam Eaton. Giolito began the 2017 season with the Charlotte Knights of the International League. On August 27, 2017, Giolito earned his first MLB victory in a 7–1 White Sox victory over the Detroit Tigers. Giolito threw seven scoreless innings yielding only three hits and striking out four Tigers. Giolito finished the season going 3-3 with a 2.38 ERA in 7 starts. The following season, in 2018, he was part of the starting rotation, making 32 starts for the White Sox, going 10-13 with a 6.13 ERA. In innings, he struck out 125 but led the American league in walks and earned runs, and was 3rd in the league in hit by pitch, 4th in wild pitches, and 9th in home runs allowed. In 2018 he had the highest rate of bases on balls per 9 innings pitched in the majors, and had the highest WHIP among major league pitchers. He also had the worst left on base percentage of all qualifying major league pitchers, stranding only 63.5% of base runners. In 2019, Giolito's third season and second in the White Sox starting rotation, he greatly improved from the previous year as he went on a nine game winning streak after starting the year with a 1-1 record. He pitched his first career shutout on May 23rd against the Houston Astros. He was also rewarded AL pitcher of the month for the month of May. His streak ended on June 19th when he gave up six runs against the Chicago Cubs, receiving only his second loss. On June 30, 2019, Giolito was named as one of the American League pitchers for the 2019 MLB All-Star Game, his first All-Star selection. On September 16, 2019, the White Sox shut down Giolito for the rest of the season due to a mild lat strain in his pitching arm. Giolito finished the season with a 14-9 record, a 3.41 ERA, 228 strikeouts and three complete games over innings in 29 starts.
