He was a member of the French National Assembly for Allier from 1978 to 1993, then from 1997 to 2002, and was president of the Communist caucus in the Assembly from 1981 to 1993.
A close collaborator of party leader Georges Marchais, he was chosen to be the PCF's candidate in the 1988 presidential election. A rather mediocre public speaker, Lajoinie proved to be an uncharismatic candidate, and was lampooned by humorists who caricatured him as a dimwit. His voting share of 6.8% was considered an unusually mediocre result for PCF standards. Lajoinie retired from active politics in 2002.