George Duncan was born in India of missionary parents but brought up in Scotland. Educated at Merchiston Castle School and Edinburgh University, he studied for the ministry at Tyndale Hall, Bristol. He served as curate at Broadwater Parish Church, Worthing, later ministering at St James's, Carlisle; St Thomas's English Episcopal Church, Edinburgh; and Christ Church, Cockfosters. Returning to Scotland he ministered at Portland Church, Troon, for six years, before moving to St George's-Tron Church, Glasgow, after the death of Tom Allan; he ministered here from 1965 to 1977 In retirement he was a member at the Edinburgh Barclay Church.
From 1947 he was a well-known speaker at the annual Keswick Convention; and also spoke regularly the Filey Christian Holiday Crusade.