As a consequence, in 1963 he entered the University of Chicago Divinity School's history of religion program led by Eliade, receiving the Ph.D. in 1967 after a final year of study in Japan.
In 1967 Ellwood became a professor of religion at the University of Southern California, Los Angeles, serving there until retirement in 1997.
In 1988 he received a Fulbright Research Grant to study new religious movements in New Zealand, and spent six months there working in the national library in Wellington.
His first book, The Feast of Kingship, about the Daijōsai or Japanese imperial accession ceremony, was based on his Ph.D. dissertation.
He has sought through religion's material expression in art, architecture, rite, and practices to understand empathetically its inner meaning for adherents, always allowing for an immense range of individual responses, but recognizing also that religious forms have as it were a language of their own.
In 1965 Ellwood married Gracia Fay Bouwman, also a student in the University of Chicago Divinity School and subsequently an instructor in Evansville College, Indiana.
He has also served as a priest in the Liberal Catholic Church, Province of the United States of America, a small denomination informally affiliated with Theosophy.