William Barnard was born in Clapham, Surrey circa 1697. He was educated at Westminster School and Trinity College, Cambridge, graduating B.A. 1720; M.A. 1724; D.D. 1740. He was elected a fellow of the College in 1724 and two years later became Rector of Esher, Surrey, 1726-44. During this tenure he became acquainted with the Duke of Newcastle who made him his Chaplain, while in 1728 he was appointed Chaplain to the King. Barnard became Prebendary of Westminster, 1732-43; Vicar of St. Bride's, London, 1739-47; Dean of Rochester, 1743-3. In May 1744 he was appointed Bishop of Raphoe and three years later was translated to Derry on the 3rd March 1747.
Barnard, after returning to England on account of poor health, died in London on the 10th January 1768 and was buried at Westminster Abbey. He married Anne Stone, sister of his predecessor. Their eldest son later became Bishop of Limerick. William Barnard was responsible for the building of the Chapel of Ease and the Bishop's Palace in Londonderry. He published A Sermon preached before the Incorporated Society for Promoting English Protestant Schools in Ireland, Dublin 1752.