He has previously played in Major League Baseball (MLB) for the Atlanta Braves and Oakland Athletics.

[5] After signing, Muller was assigned to the GCL Braves, where he posted a 1-0 record with a 0.65 ERA in 27+2⁄3 innings pitched.

Muller did not play in a game in 2020 due to the cancellation of the minor league season because of the COVID-19 pandemic.

[12][13] On June 16, 2021, Muller was promoted to the major leagues for the first time after Tucker Davidson was placed on the injured list.

[14] He made his MLB debut that day, pitching one inning and allowing two runs against the Boston Red Sox.

[15] He made his first career start on June 21 against the New York Mets, allowing 1 run over 4 innings during the 4-2 loss.

[25] On November 1, 2024, he was removed from the 40–man roster and sent outright to Las Vegas, he elected free agency the following day.