In Spain, at the age of eight, on the occasion of the republican rebellion of General Villacampo, he ran to the Guadalajara railway station to hear rebels joining with children in cries of "Viva la República!"

A professional soldier, well known for his progressive and often radical ideas, Mangada began his military career in 1896 by joining the Infantry Academy, where he was commissioned as a lieutenant.

On May 1, 1900, as a young infantry lieutenant of the Sicilian 7th Regiment stationed in San Sebastián de los Reyes, a soldier who had heard him express some private views in sympathy to the proletarian celebration of May Day, denounced him to his colonel, and he was arrested.

In 1904 he began a close friendship with journalist and writer José Nakens, who constantly battled reactionaries and struggled tirelessly to achieve a Spanish republic.

In 1906, he writes, he had been promoted to captain, but to his great chagrin had to visit his new friend in a cell where Nakens had been imprisoned after having been suspected of agitation in favour of the murder of King Alfonso XIII.

Colonel Mangada played a major military and political role as a military officer in the Second Spanish Republic, where in 1932 he was the protagonist of the "incident of Carabanchel", in which, after the defeat of dictator Miguel Primo de Rivera (1870–1930) and the exile of the unpopular King Alfonso XIII, Mangada gained huge popular support by defending the democratically elected Spanish government against right-wing officers who supported the rebellious monarchist forces that were eventually to coalesce under Francisco Franco.