William Roper the second was the eldest son of John Roper (d. 1524), Attorney-General to Henry VIII, and his wife Jane (died c.1544), daughter and coheir of Sir John Fyneux, Chief Justice of King's Bench. The Ropers were an ancient Kentish family, owners of the manor of St Dunstan outside the West Gate of Canterbury, since known as the Roper Gate. He was educated at one of the English universities and the studied law at Lincoln's Inn, being called to the bar in 1525. He was appointed Clerk of the Pleas in the Court of King's Bench, a post previously held by his father, holding the post until shortly before his death. Aged about twenty-three it is thought he joined the household of Sir Thomas More, marrying Margaret, More's eldest daughter, in 1521. They lived together in Well Hall in Eltham, Kent.