He played college football at Temple, and was signed as an undrafted free agent by the Indianapolis Colts in 2017.

He also played for the Houston Roughnecks of the XFL in 2020, leading the league in passing yards and touchdowns before it suspended operations due to the COVID-19 pandemic.

Following the completion of his high school career, Walker chose to attend Temple University, under head coach Matt Rhule.

Walker would eventually earn the starting spot at quarterback in week 6 against Cincinnati, where he would throw his first collegiate touchdown.

In a rare feat, Walker would both throw for and receive a pass in two two-point conversion attempts in Week 8 in a winning effort against UCF.

[8] Walker would lead the Owls to a 10–4 overall record (7–1 in conference play), earning a bid to the Boca Raton Bowl in losing effort against the Toledo Rockets by a score of 17–32.

Additionally, Walker would receive numerous honors, accolades and watch-list nominations in the 2016 season, including American Athletic Conference Championship Game Most Outstanding Player, CFPA National Player of the Year Watch List, Davey O'Brien Award Watch List, as well as multiple preseason and conference recognitions.

Walker led the Owls to their first conference title in 49 years (2016) and their first ever back-to-back bowl appearances in school history (2015 and 2016).

The Pittsburgh Steelers called the XFL office to ask if they could sign Walker in October 2019, but was blocked as he was already under contract with the league.

[25] In the Roughnecks' first game against the Los Angeles Wildcats, Walker tossed four touchdowns, helping the team win a 37–17 victory.

[30][31] On March 25, 2020, Walker signed a two-year contract with the Panthers worth up to $1.5 million, reuniting him with former Temple head coach Matt Rhule.

With Bridgewater remaining injured, Walker made his first NFL start the following week against the Detroit Lions, throwing for 258 yards, one touchdown, and two interceptions in a 20–0 win.

[43] Walker was again elevated to the active roster and named the starter for the Browns' Week 6 game against the San Francisco 49ers following an injury to Deshaun Watson.

In Week 7, Walker entered the game again due to an injury to Watson, throwing for 178 yards and leading the Browns to a 39–38 last-minute win over the Colts.