After beginning his career as a lawyer in Brest, he married Henriette Esperaber, an associate jurist he met through the Le Sillon movement, on September 17, 1908, in Sauveterre-de-Béarn.
A French Democratic Confederation of Labour activist, Fonlupt served as a lawyer for Pierre Mendès France during World War II.
However, his opposition to Strasbourg mayor Charles Frey—against whom he regularly ran in elections—prevented him from being appointed as the regional commissioner of the Republic after the Liberation.
Instead, the Provisional Government of the French Republic assigned this role to fellow Christian democrat Charles Blondel, while Fonlupt became the prefect of Haut-Rhin.
[5] He is referenced in Aimé Cesaire's Discourse on Colonialism, in which Cesaire sardonically notes that "Fontlup-Esperaber, who starches his mustache with it [blood], the walrus mustache of an ancient Gaul," alluding to his role in French colonial violence.