Mahon was the son of Thomas Mahon and Honorary Jane, daughter of Maurice Crosbie, 1st Baron Brandon, born in Strokestown. Catherine (died March 1834), daughter of Stephen Moore, 1st Viscount Mount Cashell.They had three sons: Thomas (1766–1835), Stephen (1768–1828), and Maurice (1772–1845).
He was able to transform his support of the Union of Great Britain and Ireland into a peerage, but was frustrated in his subsequent desire to become a viscount. Maurice"s father Thomas died in 1782, and Maurice succeeded to the family"s Strokestown estate. The Mahons were one of several important electoral interests in Roscommon, and Maurice was able to succeed his father as one of the representatives for County Roscommon in the Irish Parliament.
However, he lost the seat to Arthur French, of French Park, in 1783.
Mahon was eager to be created a peer, and the widespread bribery preceding the Union provided him with an opportunity. When Viscount Kingsborough succeeded his father as Earl of Kingston in 1799 and triggered a by-election for County Roscommon, Mahon saw an opportunity.
The King family"s preferred candidate, Honorary Robert King, opposed the Union, and Mahon got the support of the administration by promising them a pro-Union vote.
King ultimately stood down before the poll, and Thomas took the Roscommon seat.
However, the Roscommon electors were implacably anti-Union, and Thomas absented himself from Parliament rather than vote for or against the Union. The stratagem worked: Stephen voted for the Union, and on 30 July 1800, Maurice was raised to the Peerage of Ireland as Baron Hartland, of Strokestown in the County of Roscommon, as a reward for his support. The administration still wished to conciliate the powerful King family, and without their support, Thomas did not stand for Roscommon in the 1802 election. in election expenses.
He made a strong showing and the Kings conceded without a poll, to the administration"s displeasure.
Stephen continued to support government on Hartland"s instructions, his object being to secure a further promotion to a viscount in the Irish peerage. He was not successful in this, although Stephen did not go into opposition until shortly after his father"s death.
