He entered the navy, and may have served aboard the 74-gun third rate HMS Kent, during her time with Admiral Sir Edward Hawke's fleet at the Battle of Cape Finisterre.
Ramillies was part of a squadron commanded by William's brother, Commodore Francis Holburne.
William Holburne took command of the 80-gun HMS Newark, which formed part of the expedition to Louisbourg under his brother in 1757.
He went to the Mediterranean in 1759, and served as part of Admiral Sir Edward Boscawen's fleet at the Battle of Lagos.
William was son to Sir James Holburn, 1st Baronet, who started the Holborn Baronetcy.