Chapuis served as vice-president of the National Union of Students of France (UNEF) from 1955 to April 1956, during which time he was responsible for union branches in the French overseas territories.
As former head of Jeunesse Étudiante Chrétienne (JEC), a Christian student youth organization, he played a major role in the shifting post-colonial focus of the UNEF.
He was the National Secretary of the Unified Socialist Party (PSU) from 26 November 1973 to 14 December 1974.
[2] His time as secretary came to an end when, alongside Michel Rocard, he defected to the Socialist Party (PS).
In 1983, he proposed a law on the creation of a parliamentary office to evaluate scientific and technological options, OPECST.