Earl of Belfast

In 1818, Lord Belfast (as he was from 1799 until 1844) was elected to the House of Commons as Member of Parliament (MP) for Carrickfergus, and two years later became representative for Belfast. In July 1830 he was sworn of the Privy Council and appointed Vice-Chamberlain of the Household in The Duke of Wellington's Tory administration. In August he was returned to Parliament for Antrim. He continued as Vice-Chamberlain after Lord Grey formed his Whig government in November 1830. In 1831 he was made a Knight Grand Cross of the Royal Guelphic Order. He remained as Vice-Chamberlain until 1834, the last months under the premiership of Lord Melbourne. In 1837 he was once again returned to Parliament for Belfast. He did not initially serve in Melbourne's second administration, but in 1838 he once again became Vice-Chamberlain of the Household. He resigned when the government fell in 1841, and during the same year he unsuccessfully contested Belfast as a Liberal candidate. He was instead raised to the Peerage of the United Kingdom in his own right as Baron Ennishowen and Carrickfergus, of Ennishowen in the County of Donegal and of Carrickfergus in the County of Antrim. He sat in the House of Lords at Westminster for three years under this title before succeeding his father in the marquessate in 1844.

Lord Donegall did not serve initially in Lord John Russell's first administration, but in 1848 he returned to the government as Captain of the Yeomen of the Guard. He resigned along with the rest of the Whig government in early 1852. Apart from his political career he was also Lord Lieutenant of Antrim from 1841 to 1883. In 1857 he was made a Knight of the Order of St Patrick. At the time of his death in 1883 he was the senior member of the Privy Council.