He was a founder of a bank in Ayr, Douglas, Heron & Company, which went bankrupt during the credit crisis of 1772.
Heron was elected unopposed at a by-election in March 1795 as the MP for the Kirkcudbright Stewartry, filling the vacancy caused by the death of Alexander Stewart.
His election was the result of a deal brokered with the support of Henry Dundas whereby he was to alternate the seat with two others.
At the general election in July 1802, he faced a contest from the Tory candidate Montgomery Stewart, son of the Earl of Galloway.
She had married John Maxwell, who changed his name to Heron-Maxwell when they inherited Mary's father's estates.