Stewart, Robert (1739–1821), 1st marquess of Londonderry , politician, was born 27 September 1739 in Dublin, eldest son of Alexander Stewart (1697–1781), landowner in Co. Down and briefly MP for Derry city (1760), and his wife Mary Stewart (née Cowan), his cousin and an heiress with a fortune estimated at over £100,000. Educated privately at Newtownards, Co. Down, he studied literature at the University of Geneva before embarking on the grand tour. He returned to Ireland in the 1760s and fell in love with Lady Sarah Seymour-Conway, daughter of the earl of Hertford (qv), a wealthy landowner with vast estates in Ulster who was appointed lord lieutenant for Ireland in 1765. Having secured Hertford's consent, the couple were married 3 June 1766 at the Chapel Royal of Dublin Castle. A few months later Stewart refused the offer of an Irish peerage from his father-in-law, who had since returned to England; Stewart was determined to enter the house of commons as MP for Co. Down.
Residing at 28 Henry St., Dublin, Stewart and his wife had two sons, Alexander and Robert (qv), but only the second survived infancy. While pregnant with her third child, Lady Sarah became ill and died 17 July 1770. Elected to the Irish house of commons as MP for Co. Down (1771–83), Robert Stewart finally achieved his ambition of entering parliament. He opposed catholic relief in 1774 and in general voted with the opposition, although he moderated his position after the arrival of the duke of Portland (qv) as lord lieutenant in 1782. He married secondly (1775) Lady Frances Pratt, eldest daughter of the 1st Earl Camden; they had three sons and eight daughters. Stewart became friends with Lord Charlemont (qv), who regarded him as one of the most honest men he had ever met. A volunteer colonel, he acted as Charlemont's ADC at the review of the volunteers on College Green on 4 November 1780. He was appointed PC on 1 November 1783. A presbyterian, he had a great influence over the presbyterian vote in Co. Down. However, in the 1783 general election he was still defeated by Arthur Hill (qv), later the marquess of Downshire, after his election was overturned. Out of parliament, Stewart benefited from his powerful Camden connection, and on 20 September 1789 he was raised to the house of lords as Baron Londonderry in the peerage of Ireland. The general election of 1790 was an expensive one for Stewart. His eldest son, Robert, decided to contest his old seat of Co. Down, and he heavily financed the campaign, which became one of the most expensive in history. Despite being legally under age his son was elected, and Stewart had to sell part of his library and some paintings to defray campaign expenses. In 1795 his brother-in-law, the 2nd Earl Camden (qv), became lord lieutenant of Ireland and Stewart was advanced to Viscount Castlereagh (1795) and then earl of Londonderry (1796).
Overshadowed by his brilliant eldest son, who received the courtesy title of Viscount Castlereagh in 1796, Stewart played little part in any of the great political questions of the day. He supported the act of union, which was directed through parliament by his son, and was aggrieved not have been rewarded with a representative peerage in the house of lords of the United Kingdom in 1801. On 13 January 1816, as a consequence of his son's glittering career as foreign secretary, Stewart was created marquess of Londonderry. Involved in much of the rebuilding of the house at Mount Stewart, Co. Down, he also oversaw the building of a ‘Temple of the Winds’ on a hill over Strangford Lough. It is regarded as one of the finest neo-classical buildings of its kind in Ireland.
Stewart died 6 April 1821 at Mount Stewart and was buried at Bayham Abbey, Kent. He was succeeded as 2nd marquess of Londonderry by his eldest son, Robert Stewart, who committed suicide in 1822. His eldest surviving son from his second marriage, Charles William Stewart (later Vane) (qv) then succeeded as 3rd marquess of Londonderry.