Known at first mostly as the brother of tennis star Billie Jean King, Moffitt began to make a name for himself in the San Francisco bullpen throughout the 1970s. He signed with the Astros as a free agent in 1982 and posted a 2-4 record with three saves and a 3.02 ERA in 30 games. The next year, as a Blue Jay, was his last in the majors.

For his 12 big league seasons, Moffitt built a 43-52 record with a 3.65 record and 96 saves.