He played in Major League Baseball (MLB) for the Tampa Bay Devil Rays/Rays, Colorado Rockies, Baltimore Orioles, Oakland Athletics, Chicago Cubs, and Kansas City Royals.

He attended South Kitsap High School in Port Orchard, Washington, and led the team to a 20–1 season in 2000 as the number one starter in the rotation.

Hammel was drafted in the 23rd round by the Seattle Mariners, but opted instead to attend Treasure Valley Community College in Ontario, Oregon.

He made his major league debut on April 11, 2006, against the Baltimore Orioles and got his first victory as a starter on September 2, 2007, against the New York Yankees.

On April 5, 2009, the Rays traded Hammel to the Colorado Rockies for pitching prospect Aneury Rodríguez.

On October 11, 2009, Hammel started game three of the 2009 National League Division Series against the Philadelphia Phillies.

[10] On June 1, 2013, during a game against the Tigers, Hammel was ejected for the first time in his career, by home plate umpire Hunter Wendelstedt.

On December 8, 2014, after becoming a free agent following the end of the 2014 season, Hammel agreed to terms on a two-year, $20 million deal to return to the Chicago Cubs.

Hammel also was the starting pitcher in game four of the 2015 National League Championship Series vs. the New York Mets.

[30] They resided in the Lake View neighborhood of Chicago but sold the home in 2021 and moved to the Northeastern United States.