Son of a landed proprietor with an estate at Strokestown, he joined the British Army, serving for most of his career with the 9th Light Dragoons.
Mahon succeeded his father as Lord Hartland in 1819 and died without issue in 1835, his title and estates passing to his youngest brother.
Swift reprisals followed: martial law was imposed, and two hundred executions took place shortly after, including that of Sir Edward Crosbie.
Mahon, a pro-Unionist, gained the backing of the Lord-Lieutenant, and the Dublin Castle administration enlisted the independent and Catholic interests in the county on his behalf.
Maurice was forced to buy a seat at Knocktopher for Thomas's younger brother Stephen to carry out his obligations to the administration.
[8] Maurice was rewarded with the title of Baron Hartland after the Union, and Thomas, like other Irish county members, continued to sit for Roscommon in the Parliament of the United Kingdom.
Erris was wealthy and capable of fighting an expensive electoral contest; Hartland attempted to persuade Henry Augustus Dillon to stand alongside Thomas for Roscommon to bolster his popularity.
[7] Absent on military service for the 1806 election, he did not return to politics, leaving his brother Stephen to represent the family interest in that seat.
[12] The regiment represented the only cavalry of the army's left wing, and was so badly affected by sickness acquired there as to cripple it for the remainder of the Napoleonic Wars.
[9] Mahon and his brother Stephen were both promoted to lieutenant-general on 12 August 1819, in part to conciliate them over their lack of other military preferments, and their father's unrequited desire to be made a viscount.