He made his MLB debut with the Nationals in 2016. His fastball has been clocked as high as . 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 (UCLA) 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. Giolito signed with the Nationals on July 13, 2012—thirty seconds before the deadline—to a minor league contract with a $2.925 million signing bonus. By the end of the month, the team had scheduled Tommy John surgery to repair his injured elbow ligament. By the following summer, he made eight starts for the Gulf Coast Nationals of the Rookie-level Gulf Coast League in 2013 and posted a 2.78 earned run average (ERA). He was promoted to the Auburn Doubledays of the Class A-Short Season New York-Penn League, and allowed one earned run in the 16 innings he pitched. In 2014, Giolito played his first full season of professional baseball with the Hagerstown Suns of the Class A South Atlantic League. He went 10–2 with a 2.20 ERA and 110 strikeouts compared to 28 walks over 20 starts. He was selected as the 2014 South Atlantic League Most Valuable Pitcher and Top Minor League Prospect. He also appeared in the All-Star Futures Game. Giolito began the 2015 season with the Potomac Nationals of the Class A-Advanced Carolina League, and received a midseason promotion to the Harrisburg Senators of the Class AA Eastern League. The Nationals assigned Giolito to Harrisburg to start the 2016 season. He received a promotion to the Syracuse Chiefs of the Class AAA International League in July. 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.