In 1796 Stewart was apprenticed to a London merchant banker Simeon, with a view to a future business partnership. It does not appear that anything came of this scheme.
Stewart supported the Grenville ministry, which was his father’s line, and although his brother Lord Garlies insisted that he was not ‘engaged to the government’, was supported by them when threatened with a contest in 1806; moreover, despite some doubt about his politics after the death of his father in November 1806, owing to his brother the 8th Earl’s attachment to opposition, Stewart voted against the displacement of the Grenville ministry, and was supported by their party in his election in 1807, even at the cost of alienating their friend Lord Selkirk. He evidently regretted being at odds with his brother, and subsequently supported government.
In the autumn of 1811 Lord Galloway thought that if his brother William declined to contest Wigtownshire at the general election, Montgomery might be substituted for him, adding ‘although I am adverse to his remaining in Parliament, yet if he perseveres I cannot withold my support from him, and I am apprehensive that he will not renounce a situation though unsuitable to his health and his residence’. In January 1812, with a contest brewing in Kirkcudbright, Stewart announced his retirement from Parliament owing to the present state of his health. According to Mark Boyd, his neighbour in Scotland, his dislike of being dictated to by his family in politics once had ludicrous consequences. Boyd’s story is garbled, but he maintained that after voting for an amendment made by Canning, whose admirer he was, to a motion of Castlereagh’s, Stewart found the door barred to him on his next visit to the House, his father having applied for the Chiltern Hundreds for him, using a letter signed by Stewart after his first election. A row ensued, which resulted in Stewart’s retention of his seat. Boyd added:
But I have been told on excellent authority, that even after the death of his father ... in 1806, he was much too honest a politician to allow himself to be dictated to regarding his votes during the six remaining years of his parliamentary life, and that this feeling of independence on his part frequently led to family differences which had better have been avoided.
One part of this story, at least, has not been confirmed, that is, Boyd’s allegation that a new writ was actually issued for Kirkcudbright.
