William Bentinck was the eldest son of Captain John Albert Bentinck, of Terrington St Clement, Norfolk, and his wife Renira van Tuyll van Serooskerken. He was the grandson of the Dutch landowner William Bentinck, Count Bentinck, and a great-grandson of Hans Willem Bentinck, 1st Earl of Portland, through his second marriage to Martha Jane Temple. In 1775, on the death of his father, William Bentinck inherited Terrington St Clement.
He followed his father into the British Royal Navy. He served at the latter end of the American War of Independence, commanding the 'Assistance' in 1783, at the age of 19. His logbook records subsequent journeys in North America in 1784. He was elected a Fellow of the Royal Society in 1787. Bentinck commanded the 'Adamant' in 1793-1794 during the War of the First Coalition, and the 'Phaeton' during the Battle of the Glorious First of June in 1794. From July 1794 to March 1795 he commanded the 'Tremendous'. Between 1798 and 1802 Bentinck served as Governor of St Vincent and the Grenadines.
Bentinck's grandmother was Charlotte Sophie, Countess Bentinck (1715-1800). In 1733 she had married William Bentinck (1704-1774), Lord of Rhoon and Pendrecht and Count Bentinck. The marriage was not successful, and in 1739 Charlotte Sophie left her husband. Relations between Charlotte Sophie and her former husband became increasingly embittered and she was not permitted to see either of her two legitimate sons, despite several attempts. Both died before she could meet them again. In 1789, however, the Countess met her grandson William Bentinck for the first time. William met her at her home in Emsbuttel, Hamburg, while he was on a trip with his friend and naval colleague James Hawkins-Whitshed. Whitshed later married William's sister Sophia H. Bentinck (Sophy), with whom the Countess had an intimate correspondence until her death in 1800. The correspondence reveals Charlotte Sophie's increasing attachment to the members of her 'new' family. In her letters, William Bentinck is often referred to as 'notre meilleur Grand Guillaume' or 'le grand garçon'. In her will, Charlotte Sophie left William her castle and small estate at Doorwerth near Arnhem in the province of Gelderland, The Netherlands.
In 1802 William Bentinck married Lady Frances Eliza Augusta Pierrepont, daughter of Charles Pierrepont, 1st Earl Manvers. By 1805 Bentinck was living on his estate in Norfolk, and commanding the Sea Fencibles, a coastal defence force against the French. He was promoted to Rear-Admiral of the Blue in 1805, Rear-Admiral of the White in 1808, and Vice-Admiral of the Blue in 1810. He died of typhus in St Petersburg on 21 February 1813.