Christopher Henderson served as the Director of Field Operations one year before the events that took place in Day 1. When Jack Bauer busted Henderson for taking bribes, Henderson was fired, but he wasn't officially charged due to lack of evidence.
After the events of Day 2, Jack Bauer was reinstated to CTU on a regular basis and earned the position as Director of Field Operations. When Erin Driscoll took command of the office, she fired Jack due to his former heroin addiction. He was later reinstated when Ronnie Lobell was killed. He was later the acting Director of Field Operations in Day 6 until Mike Doyle could arrive.
Nate Burke took over as temporary Director of Field Operations after Cole was arrested for working with Jack Bauer to try to expose the Russians as part of Omar Hassan's death. When Chloe O'Brian retook her position as CTU Director she released Cole from custody and reassigned him to DoFO. Burke seemed displeased with Cole, who he thought a traitor to CTU, regaining his position.