In the years following the Revolutionary War, when people, money, and cultivated land were scarce on the Niagara Peninsula, Robert Hamilton managed to build a huge mercantile empire. When he died in 1809 aged only 56, he was owed £69,000 (or $276,000 at the rate of 4 dollars to a pound), an enormous sum for those days. He had also acquired 83,000 acres of land.

He was born in a small town in East Lothian, Scotland in 1753 to a Presbyterian minister. He was an educated man and may have, like his father and brothers, had an education at the University of Glasgow. In 1775 he joined the firm of the Ellice brothers and spent time at their office in Montreal, learning the business and making contacts that would last his lifetime. One of the most important contacts was Isaac Todd of the firm Todd & McGill, a supply house in Montreal. In 1780, Hamilton became a partner with Richard Cartwright in the firm of Hamilton & Cartwright based in Niagara.

At that time, most trading involved barter; the only sure source of money was trade with the military, who, until 1796, still occupied Fort Niagara on the east side of the Niagara River. Hamilton was able to establish a good relationship with the officers because he came from the same class, knew how to deal with them, and understood their requirements. Where other merchants had to scramble to get business from the military, Hamilton & Cartwright were able to impress the officers and build a reputation for respectability and reliability. The firm used Todd & McGill as a supplier in Montreal and, in turn, became a supplier for one of Todd & McGill's oldest customers, John Askin in Detroit. Eventually, Hamilton established the business in Queenston while Cartwright built up the business in Cataraqui (Kingston).

Hamilton, besides building links to his suppliers, began to build personal links. He brought his cousins over from Dumfries in Scotland to learn the business and eventually start businesses of their own. Their names were Robert, Thomas, and William Dickson, and Thomas Clarke. Robert Dickson eventually established himself in the fur trade, and Thomas Dickson had his own retail business. William Dickson also became a merchant, but then became a lawyer, land speculator, and developer. He bought Block 1 of the former Six Nations Reserve and founded the city of Galt. Thomas Clarke became a partner of Samuel Street Jr. in milling at Bridgewater near Niagara Falls, Ontario. Like William Dickson, he became a land speculator and his wealth enabled his partner's son Thomas Clarke Street to become Upper Canada's first millionaire.

Richard Cartwright also used cousins in the business. One cousin he helped was Richard Beasley, one of the founders of Ancaster and the owner of the land upon which Dundurn Castle was built. Cartwright married Magdalen Secord, sister of James Secord, the husband of the famous Laura Secord.

Hamilton's first wife was Catharine Askin Robertson, the daughter of his business client, John Askin of Detroit. The City of St Catharines was named after her. One of his sons was George Hamilton, a founder of the City of Hamilton.