Isaac Todd was born about 1742 in Coleraine, Londonderry County, Ireland.

He was one of Montreal's most prominent merchants. He spent his early manhood as a merchant in Ireland. After the British Conquest of New France, he quickly seized upon the fur trade opportunities now available in Canada. He was established in Montreal by 1764, and his ready access to capital and patronage through his family at home no doubt aided his early ventures there. Trading at Michilimackinac made Todd aware of opportunities opening beyond Grand Portage, and starting in 1767, he sent wintering clerks into the Northwest, who traded with considerable success. By the early 1770s, Todd invested large amounts in the trade beyond Grand Portage, cooperating with other Montreal traders. He became a partner of James McGill. Their company, Todd, McGill and Company, was founded in 1776. Todd and McGill continued to participate in the Northwest trade through the American War of Independence. When the North West Company was organized in 1783-84, they were not included, choosing instead to concentrate on the still lucrative "Southwest trade" out of Lake Michigan into the Mississippi valley. This trade continued until the posts of Detroit and Michilimackinac were surrendered to the Americans in 1796. Todd remained active in Montreal’s public life. In March 1787, he became a member of the grand jury and a captain in the British militia, but his political activities, unless directly related to commerce, declined as those of McGill intensified. In the 1790s, his and McGill’s principal commercial pursuits, conducted from warehouses on Rue Saint-Paul, continued to be related to the fur trade. They imported manufactured goods from Britain and tobacco and spirits from the West Indies to sell to other merchants and for their own business. Between 1790 and 1796, they hired at least 518 engagés or indentured servants. Fifty-seven percent of them agreed to go wherever they might be sent, a situation that allowed Todd, McGill and Company to adjust to fluctuations in an unstable field of enterprise. He was also involved in land speculation, alone and with McGill.

Todd never married, but through a relationship with his housekeeper Jane Kyle, he was the father of one daughter, Eleonor Todd. He died on May 22, 1819, in Bath, Somerset, England.