Likely born in St. Francois, Quebec around 1805, Gagnon entered the service of the Hudson’s Bay Company in 1829. He served as a middleman at Fort McLoughlin before being transferred to Fort Victoria in 1843 where he assisted in the construction of the new fort. Though no marriage is recorded in the journal, a reference to 'Gagnon's wife' occurs in April of 1848 upon her death from the combined effects of an abortion and the measles. In June of the same year, two children are noted, but not named, as Gagnon sent them to boarding school in Willamette, Oregon Territory. In addition to being a general labourer, Gagnon was frequently assigned tasks requiring carpentry skills and, as he was in charge of transporting James Douglas to Nisqually in 1850, it is evident that his skill as a canoesman was respected as well. Gagnon was assigned to the sales shop at Fort Victoria from 1850 to 1860. He died in 1865 at Victoria.