Ashe began to play tennis at the age of seven in a neighbourhood park. He was coached by Walter Johnson of Lynchburg, Va., who had coached tennis champion Althea Gibson (Gibson, Althea). Ashe moved to St. Louis, Mo., where he was coached by Richard Hudlin, before he entered the University of California at Los Angeles on a tennis scholarship. In 1963 Ashe won the U.S. hard-court singles championship; in 1965 he took the intercollegiate singles and doubles titles; and in 1967 he won the U.S. clay-court singles championship. In 1968 he captured the U.S. (amateur) singles and open singles championships. He played on the U.S. Davis Cup team (1963–70, 1975, 1977–78) and helped the U.S. team to win the Davis Cup challenge (final) round in 1968, 1969, and 1970. In the latter year he became a professional.

His criticism of South African apartheid racial policy led to denial of permission to play in that country's open tournament, and, as a consequence, on March 23, 1970, South Africa was excluded from Davis Cup competition. In 1975, when he won the Wimbledon singles and the World Championship singles, he was ranked first in world tennis. After retiring from play in 1980, he became captain of the U.S. Davis Cup team, a position he held from 1981 to 1985.

Ashe underwent coronary bypass operations in 1979 and 1983. In April 1992 he revealed that he had become infected with the virus that causes AIDS, probably through a tainted blood transfusion received during one of those operations. For the remainder of his life, Ashe devoted considerable time to efforts to educate the public about the disease.