As a young man, John came to Quebec and after working under his uncle he later took over as the head of the family business, in partnership with his younger brother, Mathew (1762-1820), of Glendishrock, Ayrshire. The MacNiders ran a large import and export business with five stone-built warehouses on Rue de la Fabrique in Quebec City. Their firm sold Canadian timber and supplies to the Royal Navy and traded in wine and spices from Europe and the British West Indies to sell both at Quebec and Britain. John kept shipyards (where he built his ships and sent his timber cargoes to Britain) outside Sillery. Near there, off the Chemin de St. Louis, John built a country estate in 1802, naming his new home Kilmarnock Manor, which apart from a few obvious extensions still stands today.
