[1] The Seattle Mariners selected Williamson in the second round of the 2019 Major League Baseball draft and he signed.

[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.

[8] On March 14, 2022, the Mariners traded Williamson, Justin Dunn, Jake Fraley, and a player to be named later (Connor Phillips) to the Cincinnati Reds in exchange for Jesse Winker and Eugenio Suárez.

[12] 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.

On September 18, it was announced that Williamson had suffered a full tear of his ulnar collateral ligament and would undergo Tommy John surgery.