Caesar was born in Rome to a well-known patrician family (gens Julia) which supposedly traced its ancestry to Julus, the son of the Trojan prince Aeneas, who according to myth was the son of Venus. At the height of his power in 45 BC, Caesar began building a temple to Venus Genetrix at Rome, signifying his link to the goddess. His father and namesake, Caius Julius Caesar, achieved the rank of praetor (see cursus honorum). His mother was an Aurelia from the Cottae branch, a rich and influential family of plebeian stock. As a young boy, he lived in a modest house in the Subura quarter, where he apparently learned to speak several languages, including Hebrew and Gallic dialects.

The Julii Caesarii, although of impeccable aristocratic patrician stock, were not rich by the standards of the Roman nobility. Due to this, no member of his family had achieved any outstanding prominence in recent times, though in his father's generation there was a renaissance of their fortunes. His paternal aunt, Julia, married Gaius Marius, a talented general and reformer of the Roman army. Marius was also the leader of the Populares faction of the Senate, frequently opposed to the Optimates conservatives.

Towards the end of Marius' life in 86 BC, internal politics reached a breaking point. Several disputes of the Marius faction against Lucius Cornelius Sulla led to civil war and eventually opened the way to Sulla's dictatorship. Caesar was tied to the Marius party through family connections. Not only was he Marius' nephew, he was also married to Cornelia Cinnilla, the youngest daughter of Lucius Cornelius Cinna, Marius' greatest supporter and Sulla's enemy.

Thus, when Sulla emerged as the winner of this civil war and began its program of proscriptions, Caesar, not yet 20 years old, was in a bad position. Sulla ordered him to divorce Cornelia in 82 BC, but Caesar refused and prudently left Rome to hide. Only the intervention of his family and closest friends saved him from certain proscription and death. Despite Sulla's pardon, Caesar did not remain in Rome and left for military service in Asia and Cilicia. During these campaigns he served under the command of Lucius Licinius Lucullus and distinguished himself for bravery in combat. In 81 BC he was sent to Bithynia to raise a fleet with such success that his oponents in Rome spread the rumour that while there he had became the lover of King Nicomedes. His sexual escapades were such that (according to Suetonius), the elder Curio in one of his speeches called him "every woman's man and every man's woman."

Back in Rome in 78 BC, when Sulla died, Caesar began his political career in the Forum at Rome as an advocate, known for his oratory and ruthless prosecution of former governors notorious for extortion and corruption. Aiming at rhetorical perfection, Caesar traveled to Rhodes for philosophical and oratorical studies with the famous teacher Apollonius Molo.

On the way, Caesar was kidnapped by pirates. When they demanded a ransom of twenty talents, he laughed at them, saying they did not know who they had captured. Instead, he ordered them to ask for fifty. They accepted, and Caesar sent his followers to various cities to collect the ransom money. Thirty-eight days later, they returned with the ransom and Caesar was set free. As soon as he was ransomed, he organized a naval force, captured the pirates and their stronghold and put them to death by crucifixion. However, since he admired them, he had their throats cut before they were crucified.

In 69 BC, Caesar became a widower after Cornelia's death trying to deliver a stillborn son. In the same year, he lost his aunt Julia, to whom he was very attached. During the funerals Caesar delivered eulogy speeches from the rostra. Julia's funeral was filled with political connotations, since Caesar insisted on parading Marius's funeral mask. This was the first attack on the Sullan proscription laws of the former decade. Although Caesar was very fond of both women (according to Suetonius), these speeches were interpreted by his political opponents as propaganda for his upcoming election for the office of quaestor.