He, his widowed mother and his brothers left for Barcelona, where García Vivancos became active in radical politics. Resenting the violence with which trade unions were met by employers, he formed the anarchist group Los Solidarios, together with Buenaventura Durruti, Francisco Ascaso, Juan García Oliver, Gregorio Jover, Ramona Berri, Eusebio Brau, Manuel Campos, and Aurelio Fernández.
García Vivancos was active during the Spanish Civil War, leading the Aguiluchos Column on the Huesca Front, as well as other major units in Belchite and Teruel.
After the war, he took refuge in France, being detained in Camp Vernet. Still present there at the beginnings of Vichy France, he was freed by the Maquis and joined the French Resistance.
García Vivancos was living in Paris at the end of World War II, taking on various jobs. He started painting landscapes of the city, which he would usually sell to United States soldiers. He discovered his talent, and soon improved his technique in oil painting.
In 1947 he met and was well received by Pablo Picasso. Picasso arranged for him to meet art dealers, and García Vivancos was able to open his very own exhibition in 1948. His works are exhibited throughout the world, and form part of prestigious collections such as those of François Mitterrand, David Rothschild, Helena Rubinstein, and Greta Garbo.