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.

[20] Sims faced the Los Angeles Dodgers on August 1, 2017, in his major league debut.

[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.

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] On July 30, 2018, the Braves traded Sims, Matt Wisler, and Preston Tucker to the Cincinnati Reds in exchange for Adam Duvall.

Sims earned some notoriety for his actions during a home game against the Arizona Diamondbacks in April 2021.

[31] On July 2, Sims underwent season-ending surgery to address a herniated disc in his back.