A student of classics at Collège Saint-Laurent (now known as Cégep de Saint-Laurent) in Montréal, Quebec, Robert Desjarlais found his inspiration to become a fencer from the "cloak and dagger" novels that he read both inside and outside of his studies. In the late 1920s he and several friends formed "Les Quatre Mousquetaires" (The Four Musketeers), an amateur fencing club in Montréal, which rapidly grew in popularity during the 1930s alongside a broader national interest in the sport. Desjarlais developed as well, winning his first amateur foil competition in the city in 1939, and emerging as provincial foil and sabre champion in 1946. By the end of the year he was elected president of the Quebec Fencing Association (now the Fédération d'escrime du Québec). His efforts earned him a trip to the 1948 Summer Olympics as a member of the foil, épée, and sabre teams, but he never advanced beyond the quarterfinals with any of them. He found more success at the 1950 British Empire Games alongside George Pouliot and Edward Brooke, placing second in the team sabre event, and third in the team foil and épée competitions. Individually he placed sixth in foil and seventh in épée and sabre.
By 1954 he had retired from active competition and was the co-trainer and competition armourer for the Canadian delegation at that year's Empire Games. Soon afterwards he founded the Académie des Armes du Québec to train future competitors and coaches and ran the Palestre Nationale fencing hall in Montréal from 1958 to 1966. In 1967 he helped organize that year's FIE World Championships in Fencing that were held in Montréal and also had an association with the Commonwealth Fencing Federation. Outside of sports he was a lecturer in dramatic arts at the Université de Montréal, Université Laval, and the Conservatoire d'Arts dramatiques in Québec City and Montréal. He was inducted into the Quebec Amateur Sports Hall of Fame in 1974 and the Quebec Fencing Hall of Fame in 2008 as a builder.