Montgomerie was a man of cultivated tastes. He was especially fond of music, playing on the ’cello and composing a number of popular airs. He was the heir of the 11th Earl of Eglintoun, but as a Member of the Parliament of 1784, he occasionally found that his own opinions clashed with those of the earl. He had vacated his seat on being appointed inspector of military roads and during his term of office he greatly improved and extended the roads in the Highlands.
An agreement in 1784 between Henry Dundas and Eglintoun reserved Ayrshire for Sir Adam Fergusson in 1790. Montgomerie, acting on behalf of Eglintoun, drew Dundas’s attention to an opposition, led by Lord Cassillis, arising in the county, but added: ‘You may depend I will do everything in my power’.
As 12th Earl of Eglintoun, he acquiesced in Dundas’s suggestion that Fullarton should succeed him as county Member and again supported him in 1802. In return Dundas helped him to keep at bay Lord Cassillis’s pretensions in 1803, when Eglintoun secured the return of Sir Hew Dalrymple Hamilton.
