Walker attended Arlington High School in Arlington, Washington. Playing for the high school baseball team, as a freshman he was named All-Wesco Second Team as a utility player, as a junior he was named to the All-Wesco First Team as an infielder and to the Second Team as a pitcher, and in his senior year he was again named All-Wesco First Team pitcher as an infielder.[1]
He then attended Washington State University, where he earned a bachelor's degree in criminal justice and played college baseball for the Washington State Cougars.[2] As a freshman in 2015, he pitched in 25 games, and was 1–1 with a 2.72 ERA.[1] As a sophomore in 2016, he pitched in 18 games and was 6–3 with 5 saves (10th in the conference) and a 2.40 ERA (fourth-lowest in the Pac-12), and was named All-Pac-12 Conference honorable mention and Pac-12 All-Academic honorable mention.[1][3] As a junior in 2017, he pitched in 21 games and was 5–5 with one save and a 5.67 ERA, and was named Pac-12 Conference All-Academic honorable mention.[1] As a senior in 2018, he pitched in 23 games out of the bullpen and was 4–4 with 5 saves (9th in the conference) and a 3.98 ERA, as he struck out 47 batters in 46.1 innings, and was named Pac-12 Conference All-Academic honorable mention.[1]
He spent the entire 2019 season with the Single-A Augusta GreenJackets. He pitched in 37 games (8th in the South Atlantic League) and logging a 3.36 ERA with 61 strikeouts and seven saves in 59 innings of work.[6] Walker did not play in a game in 2020, due to the cancellation of the minor league season because of the COVID-19 pandemic.[7]
Walker returned to action in 2021, playing for the High-A Eugene Emeralds and Double-A Richmond Flying Squirrels. In 39 appearances, he posted a 3.65 ERA with 66 strikeouts in 49+1⁄3 innings pitched.[8]
In 2022, Walker split the season between Richmond and the Triple-A Sacramento River Cats. In 50 combined appearances, Walker registered a 7–3 record and 3.74 ERA with 65 strikeouts and two saves in 53 innings pitched.[9] He used a unique deceptive crossfire delivery, starting on the first base side of the pitching rubber and then stepping towards the third-base line and throwing across his body with a low three-quarters release, throwing a 94 mph sinker and an 82 mph slider.[10]
Walker returned to Sacramento to begin the 2023 season, where he made 15 appearances and recorded an excellent 0.89 ERA with 23 strikeouts and one save in 20+1⁄3 innings pitched. On May 19, 2023, he was selected by the Giants to the 40-man roster and promoted to the major leagues for the first time.[11] On May 21, 2023, Walker made his first major league appearance and earned his first major league victory, against the Miami Marlins.