He was educated at Eton and Oxford and afterwards travelled from 1757 until 1761, visiting Germany, Poland and Italy. In 1761-1762 he was M. P. for Wembley for the Whigs, and in May 1762 he succeeded as 3rd Duke of Portland. 
Although he inherited great wealth from both father and mother, he nevertheless brought himself almost to poverty with his financial contributions to the Whig party, for which he was disliked by King George III. At one state to support the crucial election in Carlisle, he gave 30,000 pounds to encourage the voters. 
He was, as well: Harleian Trustee of the British Museum from 1764 until he died; Privy Councillor, 10 July 1765; and Lord Chamberlain of the Household in the same year. Having married Lady Dorothy Cavendish, member of another Whig family, in November 1766, in December 1766 he 
retired with Rockingham and the other Whigs. Now followed a period of opposition in the House of Lords which caused a quarrel with the Duke of Grafton. In April 1782 the Marquis of Rockingham returned to power and the Duke of Portland became Lord-Lieutenant of Ireland from 8 April until 15 August 1782. 
On the death of Lord Rockingham, King George III wanted Lord Shelburne as Prime Minister, but Charles James Fox requested the king to choose the Duke of Portland. When the king refused, Charles James Fox resigned, followed by others. When forces combined against him, Shelburne had to resign and, even though openly disliked by the king, the Duke of Portland became Prime Minister in April 1783, together with ministers as much disliked and all knowing the king would get rid of them as soon as possible. 
The first disagreement with the king was their proposal for the allowance of the Prince of Wales, as it was twice the amount the king had received when he himself had been Prince of Wales. When the king objected, they had a heated argument and the Prince was forced to accept half the amount plus the revenue of the Duchy of Cornwall and a promise to assist in paying the Prince's debts. However, a few months later, failure of the India Bill ended the Portland's ministry and for eleven years he was out of government. 
In opposition the party tactics were decided upon by Fox and Burke while the Duke of Portland collected books and art and studied music. In 1792 he was elected Chancellor of the University of Oxford, which he remained till the end of his life. 
In 1794 he returned to Government as Home Secretary for seven years, and these would be the most important and useful in his life. By Royal license on 5 October 1801, he added the Cavendish name to his family surname. On 22 March 1806, according to Lord Granville Leveson-Gower, the Duke had been "cut for stone", being "seven minutes under the knife without a groan". 
In 1807, doubtful about his health and strength, he nevertheless again offered himself as Prime Minister and was accepted. His ministry was his in name only as he was old and feeble and could hardly read his papers, so his ministers held meetings without inviting him to attend. The real power of his government was held by Castlereagh and Canning; however, after a period they had a falling out, fought a duel, and both resigned. Now Portland's health deteriorated quickly and, in August 1809, he suffered a stroke and died in October the same year. He had been without ambition, no obvious talents, and no gift of oratory; yet he was considered to have offered something far more valuable---he was an honest Duke. 

Source: Leo van de Pas