A baker and waiter, Ascaso joined the Confederación Nacional del Trabajo (CNT) and one of its armed groups, Los Justicieros. He left for Barcelona in 1922, where the group came to be known as Los Solidarios, integrating anarchist figures such as Buenaventura Durruti, Juan García Oliver, Antonio Ortiz and Gregorio Jover. They became active in reprisals against thugs hired by businesses against trade unionists, and they carried out several attacks on banks.
In 1923, on the wake of Miguel Primo de Rivera's dictatorship, Ascaso fled to France alongside several of his comrades. Received with hostility by the French authorities, he and Durruti left for Latin America, where they became active in violence carried out by the Argentine Anarchists. Their return to France saw them arrested on the charge of having plotted the assassination of Alfonso XIII of Spain during the latter's visit to Paris. Lack of evidence prevented their conviction, but they were expelled from the country and denied future residency in other places (including Belgium, Germany, and Sweden). Ascaso nonetheless continued to live in France as a clandestine.
Immediately after the proclamation of the Second Spanish Republic in 1931, he returned to Spain and founded the revolutionary group Nosotros, one more radical than the Federación Anarquista Ibérica (FAI). He took part in anarchist insurrections that marked the first years of the Republic, and in 1932 was detained and deported in Bata, on the coast of Río Muni, and then to the Canary Islands. He did however reemerge in Seville the very next year, only to be detained yet again. In 1934 he became regional general secretary of the CNT.
He died in the Spanish Civil War, on the first day of conflict in Barcelona, during the bloody fighting over the Atarazanas barracks. Soon after, a column of the anarchist militia was named for him, the Francisco Ascaso Column.