Born in St. Johns, Arizona, he is the son of Levi Stewart Udall. He was educated at the University of Arizona, and he saw combat as a gunner in the Army Air Corps during the Italian Campaign of World War II. Stewart Udall graduated from the University of Arizona Law School in 1948, and began his own law practice in Tucson shortly thereafter.
Udall became increasingly active in public service, being elected to the School Board of Amphitheater Public Schools (District 10) in Tucson in June 1951. He became the President of Amphitheater School Board in 1952.
Stewart Udall served U.S. Representative from Arizona from 1955 to 1961 and then as Secretary of the Interior under Presidents John F. Kennedy and Lyndon B. Johnson from 1961 to 1969. Udall was largely responsible for the enactment of environmental laws in Johnson's Great Society legislative agenda, including the Clear Air, Water Quality and Clean Water Restoration Acts and Amendments, the Wilderness Act of 1964, the Endangered Species Preservation Act of 1966, the Land and Water Conservation Fund Act of 1965, the Solid Waste Disposal Act of 1965, the National Trail System Act of 1968, and Wild and Scenic Rivers Act of 1968.
He was the brother of Congressman and 1976 presidential candidate Mo Udall; he served as Mo's campaign manager during the primary election, which Mo lost to Jimmy Carter. Stewart's son Tom Udall and nephew Mark Udall are currently serving in the U.S. House of Representatives.
Point Udall, the easternmost place in the United States, was named for him.
He is the author of numerous books, including his most well known, The Quiet Crisis, published in 1963. In 1968 Udall received an LL.D. from Bates College.