Mermaz became an ally of François Mitterrand in the late 1950s and in 1971 became a member of Mitterrand's staff in the French Socialist Party.

In 1967, he was elected Deputy of Isère for the first time.

In 1981, he was appointed Minister of Transport in the first government of socialist Pierre Mauroy, before his election to Presidency of the National Assembly.

Mermaz died in Limours on 15 August 2024, at the age of 92.

This article about a Socialist Party of France politician is a stub.