In 1956, he was appointed a member of the extended Central Committee and lead the South-Seine PCF local federation, in the bastion of Maurice Thorez, the historical leader of the Party.

Indeed, he was part of the young guard of the General Secretary which participated to the strengthening of Maurice Thorez's leadership, which was covertly disputed by some members of the Politburo (Laurent Casanova and Marcel Servin).

In reaction to the riots of May 1968, in a controversial article published in the party's paper L'Humanité, Marchais showed his contempt for Daniel Cohn-Bendit by calling him a "German anarchist".

Then, faced with electoral growing of the PS at the expense of his party, he imposed a re-alignment on the Soviet Union at the end of the 1970s.

One year later, he supported the Soviet invasion of Afghanistan (1979),[7] judged the Communist governments "fairly positive", and criticized the "right-wing drift" of the Socialist Party.

In 1984, after President Mitterrand renounced the left's Common Programme and the electoral sanction in the European Parliament election (only 11% of votes) the PCF's ministers resigned from the cabinet.

An electoral decline ensued and Marchais faced internal dissent from figures such as Pierre Juquin, Claude Poperen and former ministers as Charles Fiterman.

He let André Lajoinie, leader of the Communist group in the French National Assembly, represent the party in the 1988 presidential election.

In 1994, at the 28th Congress of the PCF, he ceded his place as General Secretary to Robert Hue, although he maintained his titular role as a member of the Politburo, which was now significantly renamed the National Office.