Returning home, she became more involved with the opposition to the dictator Fulgencio Batista. A meeting with revolutionary leader Frank País led her to become a leader of the revolutionary movement in Oriente province. Espín met the Castro brothers who had relocated to Mexico after their failed armed attack on the Moncada Barracks in July 1953 and release from prison in 1955. Espin acted as a messenger between the Julio 26 Movement in Mexico and Pais back in Cuba. She then went on to assist the revolutionaries in the Sierra Maestra mountains after the 26th of July Movement's return to Cuba on the Granma yacht in November 1956.
Espíns ability to speak both Spanish and English allowed her to represent the revolutionary movement on an international scale. Pepín Bosch, an executive of the Bacardi Corporation, arranged a meeting between CIA Inspector General Lyman Kirkpatrick and representatives of the 26th of July Movement in 1957. Espín, as both a revolutionary leader and the daughter of a Bacardi executive, told Kirkpatrick that the revolutionaries only wanted "what you Americans have: clean politics and a clean police system." She also acted as an interpreter for an interview between New York Times reporter Herbert Matthews and Fidel Castro in 1957, which served the dual purpose of spreading news of the revolution and assuring Cubans and the international community that Batistas claims of Castro's death were false.