Baillie’s father was the younger brother of Thomas, 7th Earl of Haddington (to which title his own eldest son succeeded on the death of Thomas Hamilton, the 9th Earl) and changed his name on inheriting his maternal grandmother’s estates. As early as 1788 his wish to become county Member for Berwick was known, but he supported Patrick Home as long as the latter chose to retain his seat. When Home’s retirement was anticipated in December 1795, encouraged by his uncle Lord Haddington he applied for and obtained Henry Dundas’s support as a friend of government at the next election.
Baillie gave a silent support to Pitt’s government and did not oppose Addington, whose additional customs duties he allegedly supported in debate in June 1803
From 1807 Baillie supported government, though he was now reckoned a Melvillite and shared their tendency to dissatisfaction with the management of Scottish patronage. Late in 1807 a piece of local patronage he had applied for was given, through government oversight, to a political enemy and he was put in ‘a very unpleasant predicament’, so he informed Robert Dundas, who passed on the hint.
Baillie had become a reluctant attender, but he was equally reluctant to retire, as Sir John Marjoribanks found when he sounded him in November 1817, with a view to succeeding to his seat. When he retired at the announcement of dissolution in June 1818 and was accused of a ruse to return Marjoribanks, some indignation was indulged in at his expense: a few voices condemned him for it at the ensuing election. Marjoribanks had failed earlier to obtain Baillie’s resignation in his favour on condition of making way for Baillie’s son in due course and knew that Baillie’s first choice for his successor was William Hay of Duns, who declined; so he was in a position to refute such allegations and put it all down to Baillie’s indecisive procrastination.