Lagaillarde was a lawyer at Blida in Algeria, a reserve officer of the paratroopers, and an elected deputy of Algiers. He took the presidency of the Association générale des étudiants d'Alger (General Association of Alger's Students) in 1957, and also took part in the Alger insurrection of May 1958, which brought Charles de Gaulle back to power. Lagaillarde was member of the Comité de salut public which opposed Algerian independence, and occupied the Gouvernement général de l'Algérie (local colonial administration). In November 1958, he was on the electoral list Algérie française (French Algeria), and then became a leader of the insurrection during the week of the barricades in January 1960.
Lagaillarde was then detained in la Santé in Paris, and took advantage of his parole to escape to Spain (along with Jean-Jacques Susini, Jean-Maurice Demarquet, Marcel Ronda and Fernand Féral Lefevre), where he joined Raoul Salan and founded the Organisation armée secrète on 3 December 1960. Deprived of his immunity as a deputy, he was sentenced in absentia to ten years of prison in March 1961.
- "L'ancien chef de l'OAS Pierre Lagaillarde est mort" (in French). liberation.fr. 17 August 2014.
- René Monzat, Enquêtes sur la droite extrême, Le Monde-éditions, 1992, p.91. Monzat quotes François Duprat, L’Ascension du MSI, Edition les Sept Couleurs, Paris, 1972