He studied theology at the University of the South, in Sewanee, Tennessee and received a Bachelor of Divinity (BD) in 1939, and an honorary Doctor of Divinity (hon DD) in 1962.
He served as curate of All Saints Episcopal Church, Atlanta, Georgia, from 1939 to 1940, when he left to become rector of St. Peter's Episcopal Church, in Rome, Georgia, where he stayed until 1945.
In 1969, he was elected the first bishop of the newly formed Episcopal Diocese of Southeast Florida and served until he retired in 1980.
The Duncan Center in Delray Beach, Florida, is named in his honor.
She died in January 1967, and Duncan married Elaine Lucille Beisler Gaither on October 7, 1967 (Elaine is the maternal grandmother of actor Billy Crudup).