The Seattle Mariners selected Williamson in the second round of the 2019 Major League Baseball draft and he signed.[2][3] Williamson made his professional debut with the Everett AquaSox, posting a 2.35 ERA over 15+1⁄3 innings.[4] Due to the cancellation of the 2020 Minor League Baseball season caused by the COVID-19 pandemic, he did not pitch for a team, but was a member of the Mariners' 60-man player pool.[5][6] Williamson returned to Everett to start 2021 and was promoted to the Arkansas Travelers during the season.[7] Over 19 starts between the two teams, he went 4–6 with a 3.39 ERA and 153 strikeouts over 98+1⁄3 innings.[8]
After making a push for a spot in the Reds’ rotation during spring training, Williamson was optioned to Triple-A Louisville to begin the 2023 season.[13] In 8 starts, he struggled to a 2–4 record and 6.62 ERA with 27 strikeouts. On May 15, 2023, the Reds announced that Williamson would make his debut the next day as the starting pitcher against the Colorado Rockies.[14] In 23 starts during his rookie campaign, he logged a 5–5 record and 4.46 ERA with 98 strikeouts across 117 innings pitched.
Williamson began the 2024 season on the injured list with a left shoulder strain.[15] On May 8, 2024, he was transferred to the 60–day injured list.[16] Williamson was activated on September 1.[17] In 4 games (3 starts) for Cincinnati, he posted a 3.77 ERA with 12 strikeouts over 14+1⁄3 innings pitched. On September 18, it was announced that Williamson had suffered a full tear of his ulnar collateral ligament and would undergo Tommy John surgery. The procedure ruled him out for the remainder of 2024. He did not pitch in the 2025 season. [18]
