Steve Mason is a large, older man with gray eyes. He is bald and has a gray mustache. He is always seen wearing a black suit and a tie.
In the early stage of the Kira investigation, he helps L by sending twelve FBI agents to Japan in order to investigate the source of a leak from within the Kira Task Force, but he withdraws from the case after all of the agents are killed by Kira.
Following L's death, he is approached by Near and together they convince United States President David Hoope (George Sairas in the anime) to form the SPK. Mason himself is a member of the SPK and, along with several others, is killed by the Death Note while it is in the hands of the Mafia.