Giulio Andreotti was born in January 1919 in Rome to parents originally from Segni, a small town in the Ciociaria region of Lazio. His father, an elementary school teacher, died when Andreotti was three years old from complications related to his wartime service. As a result, the family lived in modest circumstances, subsisting on the small pension allocated to his mother as a war widow. To prove so, Andreotti himself later recalled that the nuns at his kindergarten in Segni "fed him earth," referring to solidified bovine blood.

From an early age, Andreotti displayed sociability, composure, and maturity. He was marked in particular by an intense religious devotion, serving Mass regularly and, from the age of fourteen, recruiting younger boys to become altar servers. His close association with the clergy, however, often exposed him to ridicule from his peers. Family influences further shaped a cautious and sceptical worldview. An elderly aunt, who still remembered the capture of Rome by the Savoy in 1870, summarised for him the political culture of her time: "Before 1870, people refused to pay taxes so as not to support the Pope; after 1870, they refused to pay taxes so as not to support those who held the Pope prisoner." A family friend, escorting him to school in Rome, pointed to the parliament building and remarked: "That is where the bad people who govern us live."

Andreotti studied at the Tasso Lyceum before enrolling in the Faculty of Law at the University of Rome. He also joined the Federazione Universitaria Cattolica Italiana (FUCI), where he came in contact with several figures who would later play central roles in the political life of the Republic, including Adriano Ossicini and Aldo Moro. Nevertheless, for much of his youth, Andreotti's primary concerns remained religious and ecclesiastical rather than political. In 1938, on the eve of World War II, he sought out a volume on the Papal fleet in the Vatican Library. The librarian, surprised, asked whether he had nothing better to do. Giulio would later learn that the librarian was Alcide De Gasperi.

Andreotti's early years were also marked by recurring health problems, particularly affecting his spine and posture. In 1941, he was exempted from military service after a medical examination concluded that he had no more than six months to live. Decades later, already a prominent political figure, Andreotti recalled telephoning the physician to inform him that he was not only still alive but had also become a minister, only to discover that the doctor himself had since passed away.