Mathew sat for County Tipperary in the Irish House of Commons from 1790 to 1792, when his election was declared invalid.
His younger brother Montague James Mathew (1773–1819) succeeded him as one of the two members of the UK parliament for County Tipperary.
He was an opponent of the Union and a supporter of Catholic Emancipation, and was also "a personal enemy of George IV" and gave evidence in favour of Queen Caroline regarding her conduct at the Court of Naples during her famous trial.
[1] Lord Landaff married Gertrude Cecilia, a daughter of John La Touche, of Kildare.
Dying intestate, his estates went to his sister, Lady Elizabeth Mathew, who died in 1842 leaving the fortune to a cousin, the Vicomte de Chabot, the son of her mother's sister Elizabeth Smyth.