Seattle Mariners Mills signed with Seattle and made his professional debut with the
Everett AquaSox before earning a promotion to the
Clinton LumberKings. In innings pitched between the two club, he went 0–2 with six saves and a 1.77 ERA, striking out 29. In 2018, he began the year with the
Modesto Nuts, with whom he went 6–0 with 11 saves and a 1.91 ERA in innings pitched and was named a
California League All-Star. At the end of the season, he was promoted to the
Arkansas Travelers, pitching to a 10.13 ERA in innings. After the season, he was assigned to play for the
Peoria Javelinas of the
Arizona Fall League where he was 1–0 with a 1.93 ERA in eight appearances. In 2019, he returned to Arkansas, going 4–2 with eight saves and a 4.27 ERA over relief innings, striking out 66. On May 1, 2021, Mills was promoted to the major leagues for the first time. He made his MLB debut that night, pitching a scoreless inning of relief against the
Los Angeles Angels. For the 2021 season with the Mariners, Mills pitched innings in which he gave up 14 earned runs, walking seven, and striking out 11. Through the end of the regular season, Mills pitched in 19 games for Kansas City, striking out 20 batters in innings with a 4.79 ERA. He also pitched in 13 minor-league games with the
Omaha Storm Chasers, posted a 2.57 ERA while striking out 23 batters in 14 innings. On December 13, Mills was
designated for assignment.
Boston Red Sox On December 16, 2022, Mills was traded to the
Boston Red Sox for minor-league pitcher Jacob Wallace. Mills began the 2023 season on the injured list due to right elbow inflammation. He suffered additional elbow inflammation in mid-March, delaying his recovery, and was transferred to the 60-day injured list on April 16. On July 5, Mills underwent
Tommy John surgery, ending his season. Following the season on November 17, the Red Sox declined to tender Mills an offer, making him a free agent. On November 19, 2023, Mills re-signed with the Red Sox on a two-year minor league contract. He returned to action in 2025 with the Triple-A
Worcester Red Sox, making 32 appearances (seven starts) in which he posted a 4-2 record and 3.12 ERA with 49 strikeouts and one save across 52 innings of work. Mills was released by the Red Sox organization on August 3, 2025.
Los Angeles Dodgers On August 16, 2025, Mills signed a minor league contract with the
Los Angeles Dodgers organization and was assigned to the Triple-A
Oklahoma City Comets. ==International career==