Mortimer was educated at Street Edward"s School, Oxford and Keble College in the same city.
He was ordained in 1927 and was a curate at Street Mary"s Redcliffe, Bristol. He then became a lecturer in canon law and then the Regius Professor of Moral and Pastoral Theology at the University of Oxford before his ordination to the episcopate as the Bishop of Exeter in 1949. He held this position for 24 years.
Mortimer was also a notable author
He had four children, one of whom is the journalist and author Edward Mortimer.