Fortney Hillman "Pete" Stark (1931-) is a long-time United States Representative; he has served in the US House of Representatives since 1973.
He is the winner of the Secular Coalition for America's "Find an Atheist, Humanist, Freethinker Elected Official" contest to find the highest-ranking elected US official with nontheistic beliefs.
He called himself "a Unitarian who does not believe in a Supreme Being." 
He does not call himself an atheist, but rather a nontheist. But his beliefs may fall within this site's definition of "atheist".