Selwyn was born at Church Row, Hampstead, the second son of William Selwyn (1775–1855). At the age of seven, he went to the preparatory school of Dr. Nicholas at Ealing, where the future Cardinal Newman, and his brother Francis were among his schoolfellows. He then went to Eton, where he was distinguished both as scholar and athlete, and knew William Ewart Gladstone. In 1827 he became scholar of St. John's College, Cambridge. He came out second in the classical tripos in 1831, graduating B.A. 1831, M.A. 1834, and D.D. per lit. reg. 1842, and he was made a fellow of his college. He was a member of the Cambridge crew which competed in the first Oxford and Cambridge Boat Race at Henley on Thames in 1829, losing to Oxford.