Edmonstone came of an ancient Scottish family possessing property in Ireland. This had been sold by 1783 and he increased his estate in Stirlingshire. He also invested in East India Company stock. The major part of his parliamentary career was over by 1790. He had transferred his allegiance to successive administrations and had been among the first to go over to Pitt in 1784. His connexions were with the Duke of Argyll, who supported him in Dunbartonshire in 1790, but he owed his return primarily to the Montrose-Elphinstone alliance in that county. His election was satisfactory to Henry Dundas.
He supported Pitt’s ministry in the Commons and either voted against or was absent but hostile to relieving Scotland from the Test Act, 10 May 1791. He is known to have spoken only once more, querying the purport of the motion for the abolition of the slave trade, 19 Apr. 1791. He retired in 1796 and in doing so wrote a dignified complaint to Pitt of his failure to find a living for his younger son, George, for whom he had applied to the minister since 1788:
I have now closed my political life, after 35 years, uninterruptedly, in Parliament, without any trouble, or expense to government; and may perhaps never again have the honour of being seen by you; but my attachment to your administration is well known, and whether I ever became personally disagreeable to it, I will not make myself the judge. I was certainly (upon one subject) a solicitor for eight years; and should you have the goodness to still carry your kind intentions into effect, I have, now, but my gratitude to return, which shall ever be felt, and testified:—if otherwise, I can only repent (and with real unhappiness) that I’ve been the innocent cause of leading a worthy son into the line of a profession, which, at the age of 31, does not produce him the bread of a gentleman.
In 1798 Edmonstone was still applying to Pitt on this point, ‘the only request I ever made’.