Simcoe was the only surviving son of Cornishman John (1710–1759) and Katherine Simcoe (d. 1767). His parents had four children, but he was the only one to live past childhood; Percy drowned in 1764, while Paulet William and John William died as infants. His father was a captain in the Royal Navy who commanded the 60-gun HMS "Pembroke" during the Siege of Louisbourg (1758), with James Cook as his sailing master. He died of pneumonia on 15 May 1759 on board his ship in the mouth of the Saint Lawrence River a few months prior to the siege of Quebec, and was buried at sea. The family then moved to his mother's parental home in Exeter. His paternal grandparents were William and Mary (née Hutchinson) Simcoe.