Massonnet began his career in the 1930s working as an industrial draughtsman for various companies. During the Second World War, he was captured by the Germans and imprisoned in a concentration camp in Germany. After his release, he continued his career as a designer.
In the 1950s and 1960s, Massonnet designed furniture for major French companies such as Airborne, Steiner, and Mobilier International. He is particularly known for his tubular steel chairs and armchairs, which were very popular at the time.
In addition to his furniture work, Massonnet also worked on interior architecture projects for public and private buildings, including the Roissy-Charles-de-Gaulle airport in Paris.
Massonnet received numerous awards for his work, including the gold medal from the Académie d'architecture in 1958 and the silver medal from the Brussels World Fair in 1958.