On July 30, 2015, the Astros traded Hader, Brett Phillips, Domingo Santana, and Adrian Houser to the Milwaukee Brewers for Carlos Gómez and Mike Fiers. The Brewers assigned Hader to the Biloxi Shuckers of the Class AA Southern League. He finished the season with a 3.03 ERA combined. The Brewers invited him to spring training in 2016. Hader began the 2016 season with Biloxi, and made his first start for the Colorado Springs Sky Sox of the Class AAAPacific Coast League on June 12. He was selected to appear in the 2016 All-Star Futures Game. The Brewers added him to their 40-man roster after the season. Hader began the 2017 season with Colorado Springs. The Brewers promoted him to the major leagues on June 9. He made his major league debut on June 10. Hader began 2018 in the Milwaukee bullpen. On April 30, Hader became the first pitcher ever to record eight strikeouts in an outing that was less than three innings long. He recorded a -inning save against the Cincinnati Reds, in which he faced nine batters, recording eight strikeouts and one walk. Owning a 1.21 ERA with 83 strikeouts in 44 innings, Hader was named to the 2018 MLB All-Star Game. Hader appeared in the 2018 All-Star Game at the top of the eighth inning. During his appearance, he gave up hits to Rangers'Shin-Soo Choo and Astros' George Springer before Mariners'Jean Segura hit a home run to left field to give the American All-Stars a 5–2 lead. After giving up a single to Boston Red Sox baseman Mitch Moreland, Hader was replaced after 26 pitches, allowing four hits and one out over five batters. In 2018, Hader was 6–1 with 12 saves and a 2.43 ERA. He had 143 strikeouts in innings. Among MLB pitchers who threw at least 20 innings, Hader held left-handed batters to the lowest batting average. He won the National League Reliever of the Year Award. Hader began the 2019 season as the Brewers' closer following injuries to fellow relievers Jeremy Jeffress and Corey Knebel. On March 30, Hader pitched an immaculate inning to earn the save against the St. Louis Cardinals, striking out Tyler O'Neill, Dexter Fowler, and Yairo Munoz on nine consecutive strikes, seven of which were swinging strikes. In the 2019 National League Wild Card Game, Hader blew the save for the Brewers, giving up a three-run single to Juan Soto of the Washington Nationals, which, coupled with an error from right fielder Trent Grisham, led to Washington's go-ahead run scoring on the play and resulted in the Brewers eventually losing the game. Hader finished his 2019 season with a 3-5 record, a 2.62 ERA, and 138 strikeouts over relief innings. He won his second consecutive NL Reliever of the Year Award.
During the 2018 Major League Baseball All-Star Game, a series of tweets by Hader ranging from mid-2011 to late-2012 were discovered to contain racist, homophobic and sexist content. Friends and family of Hader who were invited to the game were given blank jerseys to wear due to the controversy. Hader issued an apology after the game and deleted his account on Twitter.
Pitching style
Hader has a three-pitch repertoire: a four-seam fastball that tops out at 98 mph, a slider and a very rarely-thrown changeup. As a left-handed sidearm pitcher with great disguises to hide the ball, he is known to throw most of his fastballs and sliders in a crossfire against right-handers. This style makes Hader an extreme fly-ball pitcher.