Castro was born out of wedlock at his father's farm on 13 August 1926. His father, Ángel Castro y Argiz, was a veteran of the Spanish-American War. At the age of 6 Castro was sent to live with his teacher in Santiago de Cuba, before being baptized into the Roman Chathoic Church at the age of 8. Being baptized allowed Castro to attend the La Salie bording school in Santiago, where he regularly misbehaved; he was next sent to the privately funded Jesuit-run Dolores school in Santiago.

In 1945, Castro transferred to the Jesuit-run El Colegio de

Belén in Havana. Although Castro took an interest in history, geography, and debate at Belén, he did not excel academically, instead devoting much of his time to playing sports. In 1945, Castro began studying law at the University of Havana. Admitting he was "politically illiterate", Castro became embroiled in student activism and the violent gangsterismo culture within the university. Castro became critical of the corruption and violence of President Ramón Grau's government, delivering a public speech on the subject in November 1946 that received coverage on the front page of several newspapers. In 1947, Castro joined the Party of the Cuban People (or Orthodox Party; Partido Ortodoxo). Student violence escalated after Grau employed gang leaders as police officers, and Castro soon received a death threat urging him to leave the university. However, he refused to do so and began to carry a gun and surround himself with armed friends.

"...began his career as a revolutionary with no ideology at all: he was a student politician turned street fighter turned guerrilla, a voracious reader, an interminable speaker, and a pretty good baseball player. The only ideas that appear to have driven him were a lust for power, a willingness to use violent means to get it, and an unwillingness to share it once he had it. If he had followed any example, it was that of Napoleon, not Marx".