In season 1, Mason was the District Director of the Los Angeles division of the Counter Terrorist Unit, an agency of the federal government that seeks and hunts down terrorist groups. There is initial friction between him and Jack Bauer, because Bauer had recently blown the whistle on three CTU officials that he believed were taking bribes. Later in the season, Mason is suspected of being a mole but is cleared.
By season 2, Mason had become the Special Agent in Charge of the Los Angeles office, a step down from his position in the CTU regional office during season 1. As the closest available agent, he was sent to a suspected site of terrorist activity, where he came in close contact with weapons-grade plutonium, and was given less than a day to live by the doctor who examined him.
Due to the fact that Logan became President through the 25th Amendment and appointed Gardner as his Vice President, Gardner becomes President without ever having been elected as either Vice President or President