He has previously played in MLB for the Seattle Mariners, Arizona Diamondbacks, Toronto Blue Jays and New York Mets.

Born in Shreveport, Louisiana, Walker's original love was basketball, until he began playing baseball at the age of 11.

[3] He made four appearances for the Rookie League AZL Mariners, all in relief, and went 1–1 with a 1.29 earned run average (ERA).

On September 24, 2014, Walker pitched his first MLB complete game, allowing one run in a 1–0 loss against the Toronto Blue Jays at Rogers Centre.

He followed that outing by giving up 5 runs in 4 innings against the Dodgers, causing some to question whether or not Walker was ready for the big leagues.

Walker struggled after winning five straight, going 0–1 with an 8.02 ERA, but on July 31, he threw a complete game one-hitter (his only hit was a home run by Miguel Sano) against the Minnesota Twins at Target Field.

[9] On November 23, 2016, the Mariners traded Walker and Ketel Marte to the Arizona Diamondbacks for Jean Segura, Mitch Haniger, and Zac Curtis.

The injury required Tommy John surgery, bringing Walker's 2018 season to a premature end.

[16] On August 27, 2020, the Mariners traded Walker to the Toronto Blue Jays for a player to be named later or cash considerations.

On August 29, 2020, he made his Blue Jays debut against the Baltimore Orioles, he threw six scoreless innings, allowing four hits.

[18] With the 2020 Toronto Blue Jays, Walker appeared in 6 games, compiling a 2–1 record with 1.37 ERA and 25 strikeouts in 26+1⁄3 innings pitched.

On February 20, 2021, Walker agreed to a two-year, $20 million contract with the New York Mets with a player option for the 2023 season.

[19] He was named to the 2021 Major League Baseball All-Star Game, replacing teammate Jacob deGrom.

[26] He was placed on the injured list again as of June 22 with right index finger inflammation, after posting a 5.60 ERA in his 10 starts of the season and struggling with command over and velocity of his pitches.