He played in Major League Baseball (MLB) for the Washington Nationals, Detroit Tigers, and Milwaukee Brewers.

He finished the season leading the organization in wins, strikeouts, and earned run average, and was the MILB.com Nationals pitcher of the year.

[3] Zimmermann's contract was purchased on April 20, 2009, and he made his major league debut that night, after a two-plus hour rain delay, against the Atlanta Braves.

[7] After attempted rehab, in August 2009 Zimmermann was diagnosed with a torn ulnar collateral ligament and underwent Tommy John surgery, expecting to miss 18 months.

Over four minor league levels, he started 10 games in limited play, racking up just 39.2 innings, but compiling a solid record: 1.59 ERA, 27 hits allowed, 31 strikeouts, and just six walks.

[8] On August 26 he was recalled to make his 2010 debut back in the big leagues, where Zimmermann got a no-decision in an eventual win by the Nationals over the Cardinals.

On this same day, the Nationals learned that their other young pitching phenom, Stephen Strasburg, would need Tommy John surgery and would be out for 12–18 months.

In Zimmermann's second return start, however, five days later, he pitched six shutout innings, allowing only one hit, no walks, and striking out nine, a personal best.

On May 6, 2011, Zimmermann pitched an immaculate inning versus the Florida Marlins at Sun Life Stadium.

On May 28, 2012, Zimmermann hit his first career home run off of Miami Marlins pitcher Carlos Zambrano.

In Game 4 of the 2012 National League Division Series, Zimmermann made a relief appearance in the 7th inning, his first since 2008 at A-level Potomac.

It was the second straight one-hitter by Nationals pitching in the series, marking the first occasion since 1917 that a Washington team achieved that streak, and the first time since 1900 that the Reds were victimized that way.

He finished the season with a career high in wins, going 19–9, while also posting a 3.25 ERA and 161 strikeouts in a career-high 213+1⁄3 innings over 32 starts.

He joins Fernando Valenzuela (1981), Randy Johnson (2000), Cliff Lee (2008), Zack Greinke (2009), and Ubaldo Jiménez (2010).

[28] With the 2020 Detroit Tigers, Zimmermann appeared in three games, compiling a 0-0 record with 7.94 ERA and six strikeouts in 5+2⁄3 innings pitched.

On February 18, 2021, Zimmermann signed a minor league contract with the Milwaukee Brewers organization that included an invitation to Spring Training.