William Cavendish Bentinck, the eldest son of the 2nd Duke of Portland, was born in 1738. Educated at Eton and Oxford University, in 1761 Bentinck was elected to represent the Weobley constituency. After a year in the House of Commons, Bentinck's father died and he therefore became the 3rd Duke of Portland.
In July 1765, Portland entered Lord Rockingham's Whig cabinet, where he served as lord chamberlain until the fall of the government the following year. The Duke of Portland returned to power in 1782 when Marquis of Rockingham appointed him as Lord Lieutenant of Ireland.
For a short period in 1783 the Duke of Portland became leader of the Whig administration. The Duke of Portland's government was concerned with the power of the East India Company and in 1783 Charles Fox attempted to persuade Parliament to pass a bill that would replace the company's directors with a board of commissioners. George III made it known to the House of Lords that he would consider anyone voting with the Bill an enemy. As a result of this interference, Portland's government resigned.
William Pitt, a Tory, replaced Portland as Prime Minister and held office for the next eighteen years. In 1794, Portland and a group of the Whigs entered a formal alliance with Pitt. Portland became Home Secretary and played an important role in the passing of the Act of Union in 1801. The Duke of Portland also served as Home Secretary under Henry Addington who was Prime Minister between 1801 and 1804.
When Lord Grenville resigned in 1807 over the refusal of George III to accept Catholic Emancipation, the Duke of Portland agreed to form a new administration. Now sixty-nine years old and in poor health, Portland remained in office until shortly before his death in 1809.