The son of a doctor who had treated both the elder and younger Pitts, Henry Addington studied the law before turning to politics. He came to Brasenose in January 1774 at the age of sixteen, and took his B.A. degree in 1778. He won the Chancellor's English essay prize in 1779 and was admitted to one of the University's Vinerian law scholarships in 1780. He was led into politics though his friendship with the younger Pitt, becoming an M.P. in 1784 and Speaker of the House of Commons five years later.
George III, alarmed by Pitt's policies on Ireland and Catholic Emancipation, asked Addington to form a government in 1801. He remained in office until 1804, but although he was industrious he was also colourless and did not inspire support. He continued in politics for over twenty five years, including ten years as Home Secretary, when his severe actions in combating social unrest brought him great unpopularity.
Lord Sidmouth's last speech in Parliament opposed Catholic Emancipation and his last vote was against the Reform Bill of 1832. In many ways he was typical of the country gentry who filled Brasenose when he was an undergraduate; indeed in old age he described himself as the last of ‘the port-wine faction' in politics.