He attended the Lycée Janson-de-Sailly high school and later became the fourth leader of the Union of Communist Students alongside André Senik.
Geismar is married to Sylvie Wieviorka, deputy mayor of the Parti Socialiste (PS) of the 2nd arrondissement of Paris.
He became the organization's national leader under the direction of Jean Poperen, his former history teacher at Janson de Sailly.
[citation needed] In 1965, he became deputy secretary-general of the National Union of Higher Education (SNESup), opposing the orthodoxy of the French Communist Party (PCF) and the so-called "leftist" tendency.
On 2 May 1968 Geismar became one of the leaders of May 68 with Jacques Sauvageot (vice-president of the National Union of Students of France) and Daniel Cohn-Bendit (Mouvement du 22 Mars)[8] and joined the Movement of 22 March on 8 May.