Given the role by President David Palmer, Mason was exposed to radioactive material during Day 2 and sacrificed himself to save Los Angeles from a Nuclear Bomb, Mason was demoted from Division Director to Special Agent in Charge of Los Angeles CTU due to him going behind Ryan Chappelle orders and trading Jacks life for Alexis Drazen in Day 1.
Sent agents to CTU when he became suspicious of Ryan Chappelle's disappearance (which was a result of Tony Almeida's plan). Took over CTU as Regional Director following Chappelle's death in Day 3.