Although he lived in exile for decades, mostly in France, Carrillo was a central figure in Spanish politics during much of the tumultuous 20th Century and a player in the difficult transition to democracy in the late 1970s after dictator Francisco Franco died.
Carrillo put his longevity down to continued active participation in Spanish political life, writing essays and making contributions to public seminars and a weekly nationwide radio debate well into his 90s.
"I am a politician with a sense of reality," he told Reuters in an interview, explaining his career.
"If you can say anything good about me, it's that I have lived many years and actively participated in many episodes of Spain's history," he said, presenting a documentary in 2009.
An image firmly embedded in the nation's memory is that of Carrillo and Adolfo Suarez, another of modern Spain's founders, refusing to take cover when Civil Guards opened fire in the Spanish parliament in 1981 as part of a thwarted coup.
The son of a union organizer, Carrillo's activism began in 1931, when, at the age of 15, he began reporting for the Socialist Party newspaper and joined crowds to cheer King Alfonso XIII fleeing to exile and the declaration of Spain's Second Republic.
In 1936 he joined the army to defend the Republic from a military revolt which turned into a bloody civil war lasting almost three years and ended up installing Franco as dictator.
Carrillo, by now a Communist, was named a public order official in a defense committee set up in Madrid in November that year just as rebel troops were approaching the capital and the Republican government had fled to safety in Valencia.
After Franco's forces won in 1939 with the help of Nazi Germany and Fascist Italy, Carrillo went into exile, from where he helped organize resistance to the dictatorship.
In 1960 he became general secretary of the Spanish Communist Party (PCE), and began to draw criticism from the Franco regime for his alleged role in the 1936 massacre of several thousand supporters of the military revolt, for which far-right Spaniards blame Carrillo to this day.
The supporters were evacuated from a city jail to Paracuellos, on the outskirts of Madrid, to prevent them from joining Franco's forces which came close to taking the capital, but were then killed en masse rather than incarcerated.