To begin the 2021 season, Walker was assigned to Brooklyn (now members of the High-A East) and was promoted to the Binghamton Rumble Ponies of the Double-A Northeast and the Syracuse Mets of the Triple-A East during the season.[6][7] Over 21 games (twenty starts) between the three clubs, Walker pitched to a 9-4 record, a 3.73 ERA, and 98 strikeouts over 115+2⁄3 innings.[8] He returned to Syracuse to begin the 2022 season on the injured list.[9] He made rehab appearances with the Florida Complex League Mets and the St. Lucie Mets before he was assigned to Syracuse; over 22 innings pitched for the season between the three teams, he went 3-2 with a 4.91 ERA and 38 strikeouts.[10]
Walker began the 2023 season with Syracuse, where he made nine appearances and logged a 0.68 ERA with 18 strikeouts in 13+1⁄3 innings pitched. On May 16, 2023, Walker was selected to the 40-man roster and promoted to the major leagues for the first time.[11] In 14 games for the Mets, he struggled to an 8.10 ERA with 12 strikeouts in 10 innings of work. On August 14, Walker was placed on the injured list with a right oblique strain.[12] He was transferred to the 60–day injured list on August 17.[13]
Walker was optioned to Triple–A Syracuse to begin the 2024 season.[14][15] In 10 games for the Mets, he recorded a 5.11 ERA with 11 strikeouts across 12+1⁄3 innings pitched. Walker was designated for assignment following the acquisition of Ryne Stanek on July 26.[16]
Pittsburgh Pirates
On July 30, 2024, Walker was traded to the Pittsburgh Pirates in exchange for Nicolas Carreno.[17][18] After one appearance for the Triple–A Indianapolis Indians, the Pirates designated Walker for assignment on August 9.[19] He was released by the organization the next day.[20] Walker re-signed with the Pirates on a minor league contract on August 15.[21] He elected free agency following the season on November 4.[22]
Toronto Blue Jays
On December 20, 2024, Walker signed a one–year, $760,000 contract with the Toronto Blue Jays.[23]