Rex Brothers, a left-handed pitcher, began his Major League Baseball career with the Colorado Rockies. Drafted by the Rockies in the first round of the 2009 MLB Draft, Brothers quickly ascended through the minor league ranks, making his MLB debut on June 6, 2011. Known for his powerful fastball and sharp slider, he became a reliable option out of the bullpen for the Rockies.

In his rookie season, Brothers appeared in 48 games, posting a 2.88 ERA and striking out 59 batters over 40.2 innings. His performance solidified his role as a late-inning reliever. The following year, he continued to develop, pitching in 75 games with a 3.86 ERA and 83 strikeouts in 67.2 innings. His ability to strike out batters was a key asset for the Rockies' bullpen.

The 2013 season marked a high point in Brothers' career. He took over as the Rockies' closer mid-season and excelled in the role, finishing with 19 saves. That year, he recorded a career-best 1.74 ERA over 72 appearances, striking out 76 batters in 67.1 innings. His performance earned him recognition as one of the top relievers in the National League.

However, the following seasons proved challenging for Brothers. In 2014, he struggled with control issues, leading to an increase in walks and a higher ERA of 5.59 over 74 games. The Rockies optioned him to Triple-A in 2015, where he spent most of the season working to regain his form. He returned to the majors briefly but was unable to replicate his earlier success.

In November 2015, the Rockies traded Brothers to the Chicago Cubs. He spent the 2016 season in the minors, as injuries and inconsistency continued to hinder his progress. The Cubs released him in March 2017, and he signed a minor league deal with the Atlanta Braves shortly thereafter.

Brothers made a comeback with the Braves, returning to the majors in 2017. He appeared in 27 games, posting a 7.23 ERA with 33 strikeouts in 23.2 innings. Despite the high ERA, his strikeout ability remained evident. He spent the 2018 season in the minors and was released by the Braves in August.

In 2019, Brothers signed a minor league contract with the New York Yankees but did not make it back to the majors that year. He continued to work on his mechanics and control, hoping for another opportunity.

The Chicago Cubs offered Brothers a chance in 2020, signing him to a minor league deal. He made the team's Opening Day roster in 2021, marking his return to the majors after a three-year absence. During the 2021 season, he appeared in 57 games, recording a 5.26 ERA with 75 strikeouts in 53 innings. His ability to generate strikeouts remained a valuable asset, although control issues persisted.