Though he was apprenticed to the brewing trade and became a partner in the family brewery in 1816, Molson was primarily a financier.

The family monopoly of river transport enabled him, as owner of the Swiftsure, to engage in profitable banking operations during the War of 1812, buying bills of exchange at heavy discount in Montreal and disposing of them at a profit in Quebec.

His other interests included the first Montreal water works and gas company, fire insurance and various industrial enterprises.

He succeeded his father as a life governor, vice-president and president of the Montreal General Hospital.

As chairman of the Constitutional Association he fought on the government side in the Rebellions of 1837 and was wounded; he was given the rank of Lieutenant-Colonel of the militia.