As "Comandante Eloy," he fought in Mr Castro's revolution, reaching Havana several days before his more famous comrade in 1959. Sickened by Mr Castro's pro-Moscow swing, Mr Menoyo fled to Miami in 1961 and set up the Alpha- 66 guerrilla group to overthrow the Cuban leader.
In 1965, he was captured in the Cuban mountains and spent 22 years in horrific jails, beaten so badly that he lost an eye and the hearing in one ear, and had all his ribs cracked. He was freed in 1987 and moved back to Miami.
Mr Menoyo recently became the first major Cuban exiled opposition leader to meet Mr Castro. They talked for over three hours - a possibly historic meeting which raised the hopes of moderates but brought cries of "traitor" and "Commie-lover" from hardline exiles in Miami.
In an interview in his Havana hotel, Mr Menoyo guarded details of his conversation with Mr Castro but left no doubt that he had pushed the idea of free elections, and the leader retiring.
The Miami hardliners think retirement is too good for Mr Castro and that he should face trial for the deaths of countless Cubans executed, drowned after fleeing or ravaged by hunger.
Mr Menoyo predicted civil war, anarchy and a probable US intervention in Cuba as a result of the island's deteriorating economy if a peaceful transition to democracy is not worked out soon. Mr Castro, he said, was pragmatic and too smart to end up like the Romanian leader Nicolae Ceausescu.
"He was very tranquil, courteous, very nice,'' Mr Menoyo said. ''I saw an individual of much more experience, of much more understanding, much more tolerant, very respectful, disposed to discuss like any other sincere person.
"I think we could have positive results. Why? Because the situation in this country is very grave and Fidel Castro is a very intelligent individual. He knows perfectly well that this is not a question of Marxist-Leninism but a problem of the salvation of the Cuban nation.''
Mr Menoyo believes the fact that he reached Havana first in January 1959, almost a week before Mr Castro's forces, always niggled his old comrade and may have led to the long jail term and maltreatment.
Mr Menoyo had led the 2nd National Front of Escambray in the central mountains while Mr Castro, "Che" Guevara and their 26th of July group were in the eastern Sierra Maestra. Even with the dictator Fulgencio Batista gone from the country and the revolution victorious, it took Mr Castro about a week to reach Havana, cheered in every village.
When Mr Menoyo was later captured as an anti-Castro guerrilla in January 1965, he was taken, blindfolded, to meet his old comrade. "When they took off the blindfold, I was in an immense salon with Fidel and all the senior leaders of Cuba. Fidel said 'Gallego, I knew you'd come back and I knew we'd capture you, dead or alive.' "
''Gallego'' means ''Galician'', but is often used by South Americans to refer to Spaniards in general. Mr Menoyo was born in Madrid; an older brother died fighting for the Republicans against Franco, and, after the family moved to Cuba, another brother died in an 1957 assault on Batista's presidential palace in Havana.
Mr Menoyo spent long periods in jail in his underwear because he refused to wear prison uniform. "I was in military uniform, wearing a combatant's armband when I was detained,'' he said. ''I should have been treated as a prisoner of war." He also staged many hunger strikes, often near-fatal. "If I'd retained hate for my captors, I wouldn't have been able to survive. I was thinking of the future reconstruction of my country, which is threatened with extinction via economic bankruptcy and other dangers.
"Fidel knows that we're in favour of the democratisation of Cuba, in favour of creating a mechanism of transition because Fidel is not eternal.
"Even if he was thinking of standing down, he couldn't do it at this moment, because no mechanism has been created for a transition. There could be incredible anarchy.
"In my opinion, a person who has worked so many years in the face of so many obstacles must be thinking of creating this type of mechanism of transition that would allow him to retire. Everybody has the right to retire.''