Aimé Decosse was born on June 21, 1903 at Somerset, Manitoba. Following his studies at the Grand Séminaire of Quebec, he was ordained a priest for the Archdiocese of St. Boniface on July 4, 1926.
After ministering in various capacities, he was assigned the direction of the major seminary in 1948, and in 1952 was made Vicar General.
Appointed Bishop of Gravelbourg on November 11, 1953, and consecrated in the Gravelbourg Cathedral on January 20, 1954, his installation took place at once.
At the time of Bishop Decosse's arrival, profound changes were occurring in society. School districts were amalgamated, small family farms were disappearing, the countryside was steadily losing its population and several parishes were reduced to mission status.
Bishop Decosse experienced the Vatican Council (1962 - 1965) and the subsequent Church renewal.
On May 14, 1973, Bishop Decosse resigned his See. At first planning to spend his retirement in the service of the diocese of Gravelbourg, he answered an invitation to return to his home diocese of St. Boniface later that year.