William Parker's father, Augustine Parker, had been mayor of Queenborough, Isle of Sheppey, Kent and a commander of one of the king's yachts. William Parker entered the navy about 1756 and in 1758 was on during the capture of Louisbourg in Canada and the capture of Quebec the following year. He was promoted to lieutenant in 1762. For a time he served off the coast of Newfoundland and was promoted to commander in 1763. In 1777 he went to the West Indies where he served under Byron. He served aboard various ships and as commodore and commander-in-chief on the Leeward Islands Station between 1787 and 1789. During the 1790s he served under Admiral Lord Howe. In 1794 he commanded at the Battle of The Glorious First of June and was promoted to Rear-Admiral.