After her conversion to the Anglican Church Underhill's life consisted of various religious work including writing, visiting the poor, offering spiritual guidance, the latter increasing over time, and writing. Her first important book, Mysticism, was published in 1911.

In the 1930s she became deeply interested in the Greek Orthodox Church and joined the Fellowship of St Alban and St Sergius. Although employed in the naval intelligence (Africa) department at the Admiralty during World War One, her views changed and she became a Christian pacifist in 1939. She joined the Anglican Pacifist Fellowship and received a honorary degree of Doctor of Divinity, University of Aberdeen and died at Hampstead in 1941.