William was born on May 16, 1905, in Dover, N.H. (U.S.A.), to French-Canadian parents who had immigrated from Canada to find work in the city’s cotton mills. In 1916, his family returns to Canada for a short time. He is confirmed the following year at a parish in New Brunswick. In 1920, perhaps driven by economic reasons, the Gagnon family returns to Dover. Here William and his brothers leave school to work in the mills to help support the family.
In his early 20s, he begins discerning a religious vocation. He visits the Marists and applies to enter the Oblates of Mary Immaculate in Hudson, N.H., but is refused because a medical examination reveals a kidney disorder. He then reads on a local newspaper a column about St. John of God, a 16th century Spanish saint who founded the Brothers Hospitallers to care for the sick and needy, and feels attracted. In 1930, he travels to Montreal to enter the Hospitallers as a postulant. He has to leave the order for a short time to help support his family in New Hampshire when his father is injured. He resumes his postulancy in April 1931 and receives the religious habit three months later. He makes his solemn profession on November 21, 1935. Brother Gagnon spends his first few years with the Hospitallers in Canada, where he directs a school and holds several leadership positions, including prior of Notre-Dame-de-la-Merci Hospital in Montreal, and provincial. In 1948, he resigns as provincial and is appointed prior of l’Hopital Saint-Augustin near Quebec City.
Moved by dreams of being a missionary from a young age, Brother Gagnon volunteers for overseas mission work in 1950. On December 3, he is assigned to be the prior of the order’s new foundation in Vietnam. Post-colonial Vietnam was in a state of political turmoil when Brother Gagnon and two other Hospitaller brothers, in January 1952, take charge of the Hopital du Sacre-Coeur de Bui-Chu and its dispensary, which were located about 120 miles south of Hanoi. There, the brothers build the hospital and care for victims of war. Worn out by his apostolic labors, Brother Gagnon dies on February 28, 1972. On December 14, 2015, Pope Francis declared him venerable.