He purchased the seat from John McCausland of Strabane for £2,000 after the death of the incumbent Robert Lowry when a new writ was issued for the borough on 22 October 1765.
When the new Parliament met in 1769, Mervyn Pratt, the defeated candidate, petitioned against the election of Montgomery on grounds of bribery, corruption and undue influence.
This petition was not finally determined owing to the premature prorogation of Parliament in December 1769, so Montgomery survived and continued to represent the county until his death.
He was in the inner circle of the Irish Parliament but his low-church Northern background clung to him and whilst he became a general of volunteers and supported Flood, he opposed any relaxation of the laws against Catholics.
[2] Sketches of the Members of the Irish Parliament in 1782 stated- "George Montgomery Esq., member for County Cavan has a pretty good estate, but not a foot of land in the county he represents-is brother in law of Mr Clements-but acts independently and uniformly opposed Government till the Duke of Portland arrived-a dull and tiring speaker.".