In 1771 Auckland published Principles of Penal Law, and soon became a recognized authority on commercial and economic questions. In 1772 he took up an appointment as Under-Secretary of State for the North, a post he held until 1778. He was Member of Parliament for Woodstock from 1774 to 1784 and served as a Lord of Trade from 1776 to 1782. In 1778 he carried an Act for the improvement of the treatment of prisoners, and accompanied the Earl of Carlisle as a commissioner to North America on an unsuccessful mission to bring an end to the American War of Independence. During the War, he was head of the British spies in Europe, his budget reaching £200,000 by 1778. He probably oversaw a small group of intelligence collectors for Lord Suffolk. On his return in 1779 he published his widely-read Four Letters to the Earl of Carlisle. In 1780 Auckland became Chief Secretary for Ireland, which he remained until 1782, and was admitted to the Irish Privy Council in 1780. He represented Dungannon in the Irish House of Commons between 1781 and 1783 and was Joint Vice-Treasurer of Ireland between 1783 and 1784. While in Ireland he established the National Bank. Between 1784 and 1793 Auckland was Member of Parliament for Heytesbury. He was sworn of the British Privy Council in 1784 and served as Envoy to France from 1785 to 1787 ; he was Ambassador to Spain between 1787 and 1789 and Ambassador to the Netherlands between 1789 and 1793. In 1789 he was raised to the Peerage of Ireland as Baron Auckland and in 1793 he retired from public service, receiving a pension of £2300, and was further honoured when he was made Baron Auckland, of West Auckland in the County of Durham, in the Peerage of Great Britain. During his retirement in the country at Beckenham, he continued his friendship with William Pitt the Younger, his nearest neighbour at Holwood House, who at one time had thoughts of marrying his daughter. With Pitt's sanction he published his Remarks on the Apparent Circumstances of the War in 1795, to prepare public opinion for a peace. He was later included in Pitt's government as Joint Postmaster General in 1798. He severely criticized Pitt's resignation in 1801, from which he had endeavoured to dissuade him, and retained office under Henry Addington. This terminated his friendship with Pitt, who excluded him from his administration in 1804 though he increased his pension. Auckland later served under Lord Grenville as President of the Board of Trade in the Ministry of All the Talents between 1806 and 1807. His Journal and Correspondence, published in 1861–1862, throws much light on the political history of the time.
Lord Auckland died in May 1814 and was succeeded by his second but eldest surviving son, George, who was created Earl of Auckland in 1839. Lady Auckland died in May 1818.
