Sontag served in the U.S. Army from 1943 to 1946, during the Second World War, becoming a sergeant. He graduated from Stanford University in 1949 with a B.A. (with great distinction), then attended Yale University where he earned an M.A. in 1951 and a Ph.D. in 1952.

Sontag was the Robert C. Denison Professor of Philosophy at Pomona College, where he worked from 1952 to 2009. He had also been chairman of the department (1960—67 and 1976—77) and chairman of the Committee on Honors Study (1961—70). He also served as a visiting professor at a number of institutions (Union Theological Seminary, 1959—60, Collegio di Sant'Anselmo, Rome, 1966—67, University of Copenhagen, 1972, University of Kyoto, 1974, and East-West Center, Honolulu, 1974). He was the theologian-in-residence at American Church in Paris in 1973 and Fulbright Regional American Professor, East Asian and Pacific Area from 1977 to 1978. His research interests were Existentialism, Metaphysics, Philosophy of Religion, and Philosophical Theology. Sontag was an ordained minister in the United Church of Christ. He was considered an expert on the Unification Church. In the 1970s he interviewed church founder Sun Myung Moon and church members in Europe, America, and Asia while researching for a book published in 1977.

In a 2004 interview with Pomona College Magazine, singer, actor, songwriter, and former Pomona College student Kris Kristofferson mentioned Sontag as an important influence in his life.

In 2000, Sontag offered to let a troubled student spend the night at his home. But as he drove the student to his dormitory to pick up some clothes, the student became agitated, drew a knife and stabbed Sontag twice. Sontag testifed in the student's defense at his trial in which he was found not guilty by reason of insanity. He said about the incident: "My genes lack something, I don't seem to hold grudges...I believe in restoring relationships."

Upon his retirement from Pomona College in May 2009, Sontag was awarded the Pomona College Trustees’ Medal of Merit, as "an extraordinarily magnanimous member of this community." He died of congestive heart failure on June 14, 2009. A research fellowship fund, a gate, and a theater are named after him.