Trevor Cahill's journey through Major League Baseball began when he was drafted by the Oakland Athletics in the second round of the 2006 MLB Draft. A right-handed pitcher, Cahill made his MLB debut with the Athletics on April 7, 2009. His rookie season was marked by a solid performance, as he started 32 games and posted a 10-13 record with a 4.63 ERA. His ability to induce ground balls was a notable aspect of his pitching style, relying heavily on a sinker to keep hitters off balance.

In 2010, Cahill's performance improved significantly. He became an All-Star for the first time in his career, finishing the season with an impressive 18-8 record and a 2.97 ERA. His efforts helped the Athletics remain competitive in the American League West. That year, he also finished ninth in the American League Cy Young Award voting, a testament to his effectiveness on the mound.

Cahill continued to pitch for the Athletics until the end of the 2011 season. During his tenure with Oakland, he developed a reputation for being a reliable starter, capable of delivering quality innings. In December 2011, Cahill was traded to the Arizona Diamondbacks, along with Craig Breslow, in exchange for Jarrod Parker, Collin Cowgill, and Ryan Cook.

His first season with the Diamondbacks in 2012 saw him maintain his role as a starting pitcher. Cahill recorded a 13-12 record with a 3.78 ERA over 32 starts. However, the following seasons were more challenging. In 2013, he struggled with consistency, finishing with an 8-10 record and a 3.99 ERA. Injuries and performance issues led to a difficult 2014 season, where he split time between the starting rotation and the bullpen, ending with a 3-12 record and a 5.61 ERA.

In 2015, Cahill's career took a different turn as he was traded to the Atlanta Braves. His time with the Braves was short-lived, as he was released in June after posting a 7.52 ERA in 15 appearances. Later that year, he signed with the Los Angeles Dodgers but only appeared in a few games before being released again.

Cahill found a resurgence in 2016 with the Chicago Cubs. Transitioning to a bullpen role, he contributed to the Cubs' successful season, which culminated in a World Series championship. Although he did not play in the postseason, his regular-season performance, with a 2.74 ERA in 50 appearances, was a valuable asset to the team.

The following year, Cahill signed with the San Diego Padres, returning to a starting role. He pitched well for the Padres, posting a 3.69 ERA in 11 starts before being traded to the Kansas City Royals mid-season. His stint with the Royals was less successful, as he struggled with injuries and inconsistency.

In 2018, Cahill signed with the Oakland Athletics, returning to the team where his MLB career began. He enjoyed a solid season, primarily as a starter, finishing with a 3.76 ERA in 21 appearances. His performance helped the Athletics secure a spot in the postseason, although they were eliminated in the Wild Card game.

Cahill's career continued with the Los Angeles Angels in 2019, where he faced challenges, ending the season with a 5.98 ERA. In 2020, he joined the San Francisco Giants, adapting to a hybrid role as both a starter and reliever during the shortened season, posting a 3.24 ERA in 11 appearances.

The final chapter of Cahill's MLB career came with the Pittsburgh Pirates in 2021. He signed a one-year contract with the team, but injuries limited his effectiveness and playing time. Cahill finished the season with a 6.57 ERA in 9 appearances, marking the end of his professional playing career.