Saunders was in the merchant service before joining the Royal Navy as a volunteer in 1689 on board the Portsmouth, with Captain George St Lo, and became for a short time a prisoner of war when the ship was captured in 1690. In December 1690 he joined the Ossory with Captain Tyrrell, in which he was present in the Battle of La Hougue. On 28 December 1692 he passed his examination, aged twenty-one, after serving in the navy for not quite three years. On 5 December 1694 he was promoted to lieutenant, and in January 1695 was appointed to the Yarmouth with Captain Moody. From 1696 to 1699 he was in the Pendennis with Captain Thomas Hardy. In 1700 he was in the Suffolk and in 1701, in the Coventry, again with Hardy, and in 1702 he was first lieutenant of the St George, the flagship of Sir Stafford Fairborne. He was with Sir George Rooke at Cadiz and at Vigo.