Kirkman Finlay fell heir to the firm of James Finlay & Co in 1790 at the age of eighteen and moved into the new business of cotton spinning acquiring spinning mills at Catrine, Ayrshire, in 1801 and at Doune, Perthshire, in 1806 to add to those at Deanston, Perthshire. By 1810 he was the largest exporter of cotton yarn to Europe. He seems to have successfully by-passed the obstacles placed by the Napoleonic Wars, by making use of bases in Heligoland and Malta to run the continental blockade.
He became for a time one of the most powerful of the new business elite in Glasgow. From 1812 until 1818, after having been Provost for a year, he became MP for Clyde Burghs (of which Glasgow was one) and his main concern, reflecting the attitudes to the West of Scotland business community, was to try to break the monopoly of the East India Company in trading with India and China. While India was opened up and Finlay took advantage of it, it was not until 1833 that trade with China was opened to other than members of the East India Company. Tory in his politics, he was defeated in the election of 1818 largely because of his lack of sympathy for any demands for reform of the electorate and because of revelations of his use of paid informants to try to infiltrate the radical movements which had sprung up at this time. After 1818 he sat for the "rotten burgh" of Malmesbury, but he tried again, unsuccessfully, to represent the city in 1830.
Finlay used a large part of his considerable fortune to restore the 16th century Castle Toward on the Cowal peninsula as his country home.