Montgomery was the son of Henry Montgomery, 3rd Earl of Mount Alexander, by the Honourable Mary St Lawrence, daughter of William St Lawrence, 12th Baron Howth.
[1] He was returned to the Irish House of Commons for Antrim in 1703, a seat he held until 1713.
[2] He entered the Irish House of Lords on the death of his father in 1731.
He died in February 1745 and was succeeded in his titles by his younger brother, Thomas.
The Countess of Mount Alexander died in December 1746.