After the start of the Spanish Civil War, Tinker offered his service to a Republican side, reportedly because of his aversion to fascist regime of Benito Mussolini, who was helping the Spanish Nationalists.
While in Spain, Tinker socialized with Ernest Hemingway, Robert Hale Merriman, the leader of the American Volunteers of the Lincoln Brigade and his successor Milton Wolff, who led the 15th International Brigade during the Battle of the Ebro.