Born at Paisley in Scotland, he was a younger son of William MacNider (b.1660), of Balsarock (or Balsarach), Ayrshire.
The MacNiders had held several small estates in Ayrshire since the early seventeenth century, but spurred by the conflicts between Britain and France during the eighteenth century, Mathew and his brother William (father of John MacNider and Mrs James Johnston) moved into the import-export business.
Taking advantage of the British Conquest of New France, MacNider came to the new colony around 1760, extending the reach of his business to Quebec City.
The MacNiders quickly became one of the leading mercantile families there, selling Canadian timber and supplies to the Royal Navy and trading in wine and spices from Europe and the British West Indies to Quebec, London and Scotland.
He was supported in his election bid of 1792 by his nephews (the brothers), John and Mathew MacNider (1762–1820), of Glendrishock, Ayrshire.