He was born in El Porvenir, of Atoyac de Álvarez, in the state of Guerrero. He became politically active when he studied at the Ayotzinapa Rural Teachers' College and was a leader of the local student union. In 1962 he was elected to the post of General Secretary of the Federation of Socialistic Peasant Students of Mexico. When he began work as a teacher, he also mediated problems at other schools.
When a rector of Juan Álvarez school in Atoyac demanded that all pupils wear school uniforms, Cabañas argued that some families were so poor they could hardly feed their children, not to mention buy school uniforms. The rector was fired but his supporters remained. When an 18 May 1967 strike action ended in shooting and deaths, Cabañas fled to the mountains and joined the group of Genaro Vázquez Rojas until Vázquez' death on 2 February 1972.
Cabañas led a guerrilla group, the Party of the Poor and Peasants' Brigade Against Injustice. They numbered perhaps 300 members and lived in the Guerrero Mountains. He financed his group through kidnappings and bank robberies.
The Mexican government sent 16,000 soldiers to the Sierra Madre de Atoyac Mountains to hunt him. Fifty of them died during the chase.