He sat in the British House of Lords as one of the twenty-eight original Irish representative peer from 1800 to 1821.

Castlereagh is best remembered for his tenure as Foreign Secretary of the United Kingdom from 1812 to 1822 and played an important role at the Congress of Vienna of 1814 to 1815.

[7] In 1819 Londonderry married as his second wife Frances Anne Vane-Tempest (died 1865), daughter and wealthy heiress of Sir Henry Vane-Tempest, 2nd Baronet, through which marriage substantial estates in County Durham came into the Stewart family.

[8] Lord Londonderry was succeeded in the viscountcy of Seaham and earldom of Vane according to the special remainder by his eldest son from his second marriage while he was succeeded in the Irish titles and the barony of Stewart by his son from his first marriage to Lady Catherine Bligh, the 4th Marquess.

He had a career in both Irish and British politics but is best known for his role as Secretary of State for Air from 1931 to 1935.

Lord Londonderry subsequently gained notoriety for his informal diplomatic contacts with senior members of the German government.

He made six visits to Nazi Germany between January 1936 and September 1938 meeting Adolf Hitler on a number of occasions and sympathising with some of his viewpoints.

[9] His wife Edith Vane-Tempest-Stewart, Marchioness of Londonderry, was an influential society hostess remembered for her close friendship with Ramsay MacDonald.

[citation needed] Between 1823 and 1854 and between 1872 and 1999, the Marquesses of Londonderry sat in the House of Lords as The Earl Vane.