Montgomerie was styled Lord Montgomerie from 1769, and sat as a Member of Parliament for Ayrshire off and on from 1780 to 1796.
That year he became Lord Lieutenant of Ayrshire, which post he held until his death. In 1798, having previously succeeded to the earldom through his third cousin, he was elected a representative peer and moved to the House of Lords. On 15 February 1806, he was created Baron Ardrossan in the Peerage of the United Kingdom, enabling him to sit the Lords in his own right.
He was made a Knight of the Thistle in 1814.
As large ships were unable to reach Glasgow due to silting of the River Clyde, Montgomerie promoted and partially funded the Glasgow, Paisley and Ardrossan Canal.
However funds ran out and the canal was only constructed from Glasgow to Johnstone via Paisley.
The Glasgow terminus of the canal was at Port Eglinton.
Though the wharf is now filled in, the neighbouring Eglinton Street still bears his name. Preparatory work on the canal from the new harbour created at Ardrossan was used as the basis for Glasgow Street, which is the main thoroughfare of the town.
Montgomerie was an amateur composer and cellist. His best known work is the dance tune "Ayrshire Lasses", and other composers dedicated works to him, including Thomas Arne.