He joined the Juventud Nacionalista (Nationalist Youth) and the Solidaridad de Trabajadores Vascos (Basque Workers' Solidarity) trade union.

He moved to San Sebastián, and in 1926 had to emigrate to France due to his hostility to the dictatorship of Miguel Primo de Rivera.

When the Second Spanish Republic was proclaimed he was elected secretary of the local CGT federation, and played a central role in the many strikes that took place in Gipuzkoa.

[2] In 1933 he founded the Euskadi Roja (Red Basque Country), which he wrote, printed and sold on the streets.

In 1935 Larrañaga participated in the illegal congress at which the Communist Party of Basque Country (Partido Comunista de Euzkadi) was formed.

[2] After the attempted Spanish coup of July 1936, Larrañaga was appointed war commissioner in the newly formed San Sebastián Defense Board.

[1] In July 1936 General Francisco Llano de la Encomienda commanded the Republican Army of the North, which included the forces of Catalonia and those of the northern coast along the Bay of Biscay.

At a PCE meeting on 27 July 1937 Larrañaga attacked the Basque government, creating a storm of criticism from the Republicans.

[5] He sailed from Le Havre to Puerto Plata in the Dominican Republic in the French ship De La Salle.

Jesús Larrañaga and three others were processed by a military tribunal on 19 January 1942 headed by colonel Félix Navajas García, charged with joining the rebellion and breaking the Law of State Security.