Probate was granted on 5 January 1729. Up to 1729 Alexander Montgomery shared the parliamentary patronage of Lifford, County Donegal, with the Creighton family, later the Earls of Erne. There is an agreement in the Erne papers (held in P.R.O.N.I. in Belfast) dated 1727 between Alexander Montgomery and General David Creighton about the sharing of Lifford Corporation and its representation in the Irish House of Commons, to which it sent two MP's. One of the articles of agreement was that, if Montgomery should die without a son, then his interest should pass to the Creightons. Montgomery was elected as an MP for Donegal County in the General Election held later on in 1727 so the said General David Creighton and his son and heir, Abraham Creighton, (later the 1st Lord Erne) were returned as the two Lifford MP's. General David Creighton died in 1728 so the Lifford seat was filled by Thomas Montgomery (1700– April 1761), the 1st cousin of Alexander and father of the aforementioned Major-General Richard Montgomery. On the death of the said Thomas Montgomery in 1761, full control of the Lifford seats passed to said Abraham Creighton.