Robert Stewart Castlereagh, viscount and marquis of Londonderry, a British statesman, born at the family seat of Mount Stewart, county Down, Ireland, June 18, 1709, died by his own hand at his seat of North Cray place, Kent, England, Aug. 12, 1822. He attended the grammar school at Armagh, and completed his education at Cambridge. In 1789 he was elected to the Irish parliament for the county Down, after a sharp contest, which is said to have cost his family over £25,000. In 1794 he was returned to the British house of commons, as a member for the borough of Tre-gony. In May, 1796, he was again returned to the British parliament for Orford; but relinquishing his seat in July, 1797, he was reelected to the Irish parliament, as representative of the county Down, and appointed keeper of the privy seal of Ireland. In 1796 he became Viscount Castlereagh, on the elevation of his father to the marquisate of Londonderry. In the beginning of 1798 he became chief secretary to the lord lieutenant, and an Irish privy councillor. The rebellion which invited and accompanied the landing of Gen. Humbert in 1798 was crushed by Castlereagh. It was mainly through his instrumentality that the act of union was passed.

When this measure was consummated, he quitted the Irish government, execrated by the majority of his countrymen, He represented his native county in the first imperial parliament, which assembled in February, 1801, and also in the second, which convened in September of the ensuing year. In the beginning of 1802 he was appointed a privy councillor of Great Britain, and president of the board of control. He retained that office after the retirement of Mr. Pitt, and throughout the Addington administration. In July, 1805, after Mr. Pitt's return to power, Castlereagh joined his cabinet as secretary at war and for the colonies. Having lost his seat for Down, he was returned in 1806 for Bo-roughbridge; and relinquishing his office after Mr. Pitt's death, he was returned for the following parliament, in the same year, for the borough of Plympton Earle. He now went into opposition against Fox and Grenville, and attacked their peace policy. In 1807, upon the formation of the Portland cabinet, he again became secretary at war, and was reelected by his last constituency for the parliament which met in May of that year.

While a member of this administration, he incurred in 1809 the responsibility of the ill-advised Wal-cheren expedition, in reference to which Mr. Canning, his colleague and secretary for foreign affairs, assailed him with such warmth of personality, that a duel ensued between them, and both retired from office. Castlereagh soon returned to the ministry, and assumed Canning's post, in which he gained a position so commanding, that on Mr. Perceval's death, in 1812, he was regarded as the ministerial leader in the house of commons. In November, 1812, he was once more returned for the county Down, retaining that seat in the next two parliaments, which met in August, 1818, and in April, 1820. In 1814, as British plenipotentiary, he took part in the conferences of Chatillon, and was influential in persuading the allies not to lay down their arms unless Napoleon agreed to limit France to the boundary of 1792. This Napoleon refused to do; and that great campaign was begun which ended in the capitulation of Paris and the abdication of Napoleon. At first Castlereagh would not concur, in behalf of England, in the measure by which Napoleon was permitted to retain the title of emperor and retire to Elba. After the treaty was signed, however, he reluctantly acceded to it.

He took part in the congress of Vienna, both before and during the hundred days. Subsequently he supported George IV. in his schemes for getting rid of Queen Caroline, and was the author of the harsh measures for the repression of discontent caused by general distress and dearness of provisions. The struggles of the constitutionalists in Spain and Portugal called for active interference on the part of the holy alliance, and Castlereagh was on the point of joining the congress of Verona when he fell into a state of melancholy, in which he committed suicide by opening the carotid artery with a penknife. The coroner's jury declared the act to have been committed in a state of lunacy. He had become second marquis of Londonderry, April 8, 1821. His correspondence was edited by his brother, the third marquis, in 1850.