In 1750, he became a canon at St Paul's Cathedral, and later that year became Bishop of Lichfield and Coventry thanks to the patronage of the Duke of Newcastle, who was Secretary of State.On the death of Thomas Secker in 1768, Cornwallis's friendship with the prime minister, the Duke of Grafton, resulted in his translation to Archbishop of Canterbury.On the whole, Cornwallis has generally been judged as a competent administrator but an uninspiring leader of the 18th-century church.He is considered a typical product of the time's latitudinarianism, whose lack of zeal paved the way for the differing responses of both the Evangelicals and the Oxford Movement in the early 19th century.Church records document that three additional archbishops, including Cornwallis, are likely to be buried in the vault.