Like his elder brother Thomas, Mahon was an army officer. He was brought into the Irish parliament for Knocktopher by his father in May 1800 ‘at very great expense’ to support the Union, ‘which he did effectively’.
Although Mahon continued to vote for Catholic relief, 24 Apr. 1812, 2 Mar. 1813 (pairing because of illness on 13 May), 30 May 1815, 21 May 1816, 9 May 1817 and 3 May 1819, he was otherwise a government supporter until June 1819, despite his father’s disappointment at not getting a viscountcy. In 1815 the latter directed him to complain about this, pointing out that Mahon ‘has constantly attended and supported government both in England and here without any favour from government’. He advised:
Now I would have you promise to support any measure recommended by government consistent with your duty as a county Member provided I was to be promoted to a viscount here.
In 1816 Mahon was appointed to the Irish staff, in command of the western district, and this was his excuse for absence from Parliament in January 1817. In 1819 his father died unpromoted and his brother Thomas did not fare any better. This and ‘removal from staff’ were reported by the Castle to be the reasons for Mahon’s ceasing to support government.
