Born in Bonansa in Huesca, Aragon, Maurín engaged in socialist politics from early youth and stood trial on several occasions.

After law studies, he practiced in Lleida (Catalonia), where he became affiliated with the anarchist Confederación Nacional del Trabajo (CNT, "National Confederation of Labour").

After his release, Maurín founded the Comités Sindicalistas Revolucionarios ("Revolutionary Trade Union Committees") as a Bolshevik group within the CNT.

The unified body of the FCCB and the Catalan Communist group became the mass front Workers and Peasants Bloc (BOC), with Maurín as its general secretary.

During the riots provoked by the centrist stance of the Alejandro Lerroux government in 1934, Maurín advocated the forming of united front Alianzas Obreras ("Workers' Alliances") throughout Spain (following a pattern that was proving its force in the Asturias).

In line with his views on unified workers' action, the POUM joined the Spanish Popular Front in the runup for the elections of February 1936.