Sims played the 2013 season with the Class-A
Rome Braves. He started the season in the
bullpen but was then moved to the rotation.[4][5] He finished the season 12–4 with a 2.62 ERA and 134 strikeouts in 116+2⁄3 innings. In 2013, Sims was considered the Braves best prospect according to
MLB.com.[6]
In 2014 he was 8–11 with a 4.20 ERA in Class A+.[7] He had fallen to third on the same list by the start of the 2015 season,[8] and was placed seventh on the year-end list.[9] Between three teams, he was 7–6 with a 4.37 ERA. The Braves sent Sims to the
Arizona Fall League in 2015. He opened the AFL season for the
Peoria Javelinas,[10] and also started the Fall Stars Game.[11]
Sims started 2016 as the Braves # 13 prospect according to Baseball America, and was invited to spring training for the first time.[12][13] He began the season with the
Mississippi Braves,[14] and was promoted to
Gwinnett on April 23.[15] He struggled in the
International League and returned to Mississippi on June 17.[16] Sims finished 2016 with a 7–11 record and a 4.40 ERA; his 92 walks were the fourth-highest total in the minor leagues, and more than the total of any major leaguer.[17][18][19] The Braves added him to their 40-man roster after the season.[20]
Sims faced the
Los Angeles Dodgers on August 1, 2017, in his major league debut. He pitched six innings, yielding three earned runs and recording three strikeouts.[21] The debuts of Sims and
Ozzie Albies marked the first time since 1968 that the Atlanta Braves started two players making their first major league appearance on the same day.[22] He finished the season 3–6, with a 5.62 ERA. In December 2017, Braves manager
Brian Snitker said that he might look at
Max Fried, Sims, or another pitcher as the team's fifth starter in 2018.[23] Sims spent time with the major league team in spring training, but began the 2018 season in Gwinnett.[24]
Sims started his first game as a Red on May 28, 2019, against the
Pittsburgh Pirates.[27] In 2019, he made 24 appearances for Cincinnati, posting a 2–1 record and 4.60 ERA with 57 strikeouts in 43.0 innings pitched. In 2020, Sims went 3–0 with a 2.45 ERA and 34 strikeouts in 25+2⁄3 innings over 20 games in relief. In 2021, he went 5–3 with a 4.40 ERA and 76 strikeouts in 47 innings.
In 2022, Sims appeared in 9 games for the Reds, struggling to a 9.45 ERA with 5 strikeouts in 6.2 innings pitched.[28] He was originally placed on the injured list with a back injury on May 12, 2022,[29] then transferred to the 60-day injured list on June 17.[30] On July 2, Sims underwent season-ending surgery to address a herniated disc in his back.[28][31] He returned from the injury in April 2023.[32]