Walker was drafted by the Seattle Mariners in the first round, 43rd overall, of the 2010 MLB Draft. Walker and the Mariners agreed to a deal that included an $800,000 signing bonus. He made four appearances for the Rookie League AZL Mariners, all in relief, and went 1–1 with a 1.29 earned run average. Walker was listed as one of the Mariners' top 10 prospects for the 2011 season, ranked 4th according to Baseball America. Prior to the 2012 season, he was ranked the second best prospect in the Mariners organization and the 20th best overall. Walker was selected to play in the 2012 All-Star Futures Game. After he pitched to a 5–3 win–loss record and a 3.61 ERA with 64 strikeouts in innings pitched for the Tacoma Rainiers of the Triple-APacific Coast League, the Mariners promoted Walker on August 30, 2013. Walker pitched 5 innings against the Houston Astros, and yielded just 2 hits and struck out 2. Walker was called up when rosters expanded in September 2013. He started 3 games, finishing the season with a 3.60 ERA in 15 innings. On September 24, 2014, Walker pitched his first MLB complete game, allowing only a single run in a 1–0 loss against the Toronto Blue Jays at Rogers Centre. Walker began the 2015 season as a member of the Mariners starting rotation. In his first start, at Oakland, Walker gave up 9 runs in only 3.1 innings. 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 continued to struggle, pitching more than 6 innings only once through his first 9 starts. However, Walker won 5 straight starts in June and July, improving his record from 2-6 to 7-6. 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 against the Minnesota Twins at Target Field. This time, his offense backed him up, as he won 6-1, striking out 11 only needing 100 pitches to do so. Overall, Walker finished the 2015 season with an 11-8 record despite posting a 4.56 ERA in 29 starts. Throughout the first half of the 2016 season, Walker battled with a foot injury. He was placed on the 15-day disabled list on two occasions. He began the second half of the season on the disabled list. Before the all star break, Walker had a 4-7 record despite a 3.66 ERA and 80 strikeouts in 86 innings. Walker was optioned to AAA on August 8, 2016. On September 13, 2016, Walker pitched his first career complete-game shutout in an 8-0 Mariners win over the Los Angeles Angels of Anaheim. He carried a perfect game into the sixth inning and a no-hitter into the seventh, finishing the game allowing no walks and three hits while striking out eleven batters.
Arizona Diamondbacks
On November 23, 2016, the Mariners traded Walker and Ketel Marte to the Arizona Diamondbacks for Jean Segura, Mitch Haniger, and Zac Curtis. In his first season in Arizona, Walker made 28 starts, pitching to a 9-9 record in 157 innings. On April 15, 2018, Walker was placed on the 10-day disabled list due to right forearm tightness. Two days later, on April 17, it was revealed that Walker was diagnosed with a UCL injury in his right elbow. On April 18, it was revealed that Walker had a partial tear of the UCL in his right elbow. With the injury, it required Tommy John surgery, bringing Walker's 2018 season to a premature end. Following his Tommy John surgery, he appeared in one game, the final game of the season, for the Diamondbacks. On December 2, 2019, Walker was non-tendered by Arizona and became a free agent. On February 4, 2020, Walker worked out for 20 scouts from Major League teams.
Second Stint with the Seattle Mariners
On February 12, 2020, Walker signed a one-year contract for $2 million to return to the Seattle Mariners.
Walker's father is black, and his mother is half-Mexican and half-white. Raised by his single mother, Walker helped care for his younger siblings. Walker and his wife, Heather Restrepo, welcomed their first child, son Zayden, in July 2017.
