[2] Together with John Cook and Ian Caughley, Walker started working on Team Fortress as a mod for id Software's QuakeWorld in 1996.

[3][4] Walker has played development roles in various Valve games, including Half-Life 2 and Dota 2.

[7] Walker worked on Valve's flagship virtual reality game, Half-Life: Alyx, released on March 23, 2020.

[8] Walker used Team Fortress 2 updates to research what additional features are and aren't popular.

Walker also stated that he cannot guarantee that he would keep working on Team Fortress 2 indefinitely and that at some point, he will move on to a new project.