Groseilliers was born in 1618 in Charly-sur-Marne, France, the son of Médard and Marie (Poirier) Chouart. He also had a cousin named Médard Chouart. He later called himself Sieur des Groseilliers after a farm his parents managed in Bassevelle.
At an early age Groseilliers entered the service of the Jesuits as donné or assistant. In 1637 (or 1641) he went to Canada, where he spent several years in the mission to the Hurons on Manitoulin Island and learned the Huron tongue.
Attracted by the fur trade, Groseilliers left the mission and went to Three Rivers. By his second marriage he apparently acquired property with which, added to the proceeds of his ventures in furs, he bought land, assuming the title of Sieur des Groseilliers. In the following year, 1654, he met his brother-in-law Radisson at Three Rivers and in the words of Radisson, the two formed a partnership to "travell and see countreys. " Their first journey together may have been one made between 1654 and 1656, since both appear to have been absent from Three Rivers during this interval, but it may have been that made later, between 1658 and 1660. This expedition of 1658-1660 took them to the far west and was followed (1661-1663) by a further far western journey, the record of which was lost. The journey of 1658-1660 yielded Groseilliers and Radisson an immense quantity of furs, with which they returned to New France. The expedition had been made without a license from the governor, however, and the furs were therefore confiscated, and heavy fines imposed upon Radisson and Groseilliers alike. They went to Paris to secure remission of the fines, but returned to Quebec without success. Angry and resentful toward the government of New France because of their treatment at its hands, in 1664 they took ship for Port Royal in Acadia and then for Boston in New England. Here they were fortunate enough to meet Col. George Cartwright, royal commissioner of Charles II, and before him they laid a proposal for a voyage to Hudson Bay, in the English interest. Cartwright induced them to accompany him to England. On their way all were captured by a Dutch ship, but after being landed on the coast of Spain, finally reached their destination. Through Cartwright's influence they obtained an audience with King Charles on October 25, 1666, which resulted in the promise to them of a ship with which to make a trading venture to Hudson Bay. The expedition set forth in June 1668, two vessels, the Nonsuch and the Eaglet, sailing from the Thames for Hudson Bay. In the first Groseilliers was given passage and in the second, Radisson. The Nonsuch, with Groseilliers on board, succeeded in finding the bay and reached the south shore (James Bay) on September 29. Here the adventurers were within a hundred and fifty miles of the nearest French settlements. They entered a river (the Nemisco), which they called Rupert's River after their patron, Prince Rupert, and at its mouth they built a stockaded post which they named Fort Charles. On this initial voyage Radisson did not reach Hudson Bay at all, since the commander of the Eaglet failed of an entrance and returned to England. Thither the next spring, after a winter spent in Hudson Bay, Groseilliers returned with a cargo of furs, and upon both Groseilliers and Radisson, King Charles bestowed medals and gold chains. The success of the expedition resulted in the organization of the Hudson's Bay Company which on May 2, 1670, received a royal charter. In the same year Groseilliers and Radisson accompanied Resident Agent Charles Bayly to the new field and opened a trade on the shores of the Bay. Beginning with 1674, the adventurous brothers-in-law yielded to overtures by France, their native country, and in 1681 were sent to undertake a trading expedition to Hudson Bay from New France, in the interest of the French. In 1684 and afterward they were again in the service of the English, Radisson becoming a denizen of England and Groseilliers settling down in Canada where, prior to 1698, it is thought, he died.
Groseilliers married Hélène, widow of Claude Étienne and daughter of Abraham Martin, on September 3, 1647. She died in 1651, and on August 24, 1653, Médard was married again, to Marguerite, widowed sister of Pierre Esprit Radisson.
