selected Walker in the second round of the 2012 MLS SuperDraft. Galaxy coach Bruce Arena spoke positively of Walker, telling reporters, "He is a defensive midfielder, a holding midfielder, an aggressive type of player and a good passer of the ball." Walker made his debut for Galaxy on May 26, 2012 against Houston Dynamo. He only played one half, however, and did not have another appearance for the rest of the season. In 2013, Walker logged no minutes with the Galaxy, and spent the latter half of the season on loan in the North American Soccer League. On July 26, Walker was loaned to Florida-based Fort Lauderdale Strikers. He only had one appearance for them though, as he was loaned to Carolina RailHawks just a month later on August 22. He spent the remainder of the season in North Carolina, where he made 11 appearances. Walker began to see more action with Galaxy in 2014 and 2015, earning 7 starts and 16 appearances across the two years. He also spent significant time playing for LA Galaxy II, Galaxy's newly-formed reserve team in the United Soccer League. Walker debuted in Galaxy II's inaugural match against PDL side Fresno Fuego on February 13, 2014. Walker would go on to start in 14 of 16 appearances for Galaxy II over the following two years.
On December 7, 2015, Walker was announced as one of the first eleven signings for United Soccer League side FC Cincinnati, who played their first season in 2016. Walker quickly became one of the team's primary midfielders, starting in 27 of his 28 appearances during the club's inaugural season. He remained a key player in 2017 under the new leadership of head coachAlan Koch, starting in 26 of 27 appearances and leading the team with seven assists. On September 17, 2017, Walker was named USL Player of the Week after he notched two assists and his first goal of the season in a 4–2 win against New York Red Bulls II. Following the end of the season, FC Cincinnati announced Walker as their Most Valuable Player of 2017. Walker was one of just nine players who remained with FC Cincinnati into the 2018 season, as Koch spent the offseason significantly reworking the team roster. Walker started 2018 with a concussion injury, taking two weeks off after a heavy hit on March 31 from Indy Eleven's Jack McInerney. On April 21, Walker made his return in a match against Pittsburgh Riverhounds as a 70th-minute substitute. He scored nine minutes after coming onto the pitch, earning a 2–2 draw for the club and a place on USL's Team of the Week for himself. The following week, Walker broke a 0–0 tie against Ottawa Fury with a goal shot from 50 yards out in the 68th minute. The goal was featured as the #1 play in the SportsCenter Top 10 that night, and was later picked as the USL Fans' Choice Goal of the Month for April.
Indy Eleven
In late November 2018, ahead of FC Cincinnati's move up to Major League Soccer, Walker announced that he would be leaving the club to play for USL Championship side Indy Eleven. According to Walker, FC Cincinnati had told him and other players that they may have the chance to be signed to the MLS side, but only if the club did not get better players for the same positions in the December expansion draft. Walker declined to wait in favor of a guaranteed signing with Indy Eleven, explaining, "I think for me it came down to deciding if I wanted to play or be a guy that's mentally supporting the team and helping out at training, doing all that stuff and not being a contributing piece on weekends." On December 5, 2018, Indy Eleven officially announced their acquisition of Walker, along with his FC Cincinnati teammates Paddy Barrett and Tyler Gibson.
