Pansa was the son of moneyer Gaius Vibius Caii filius Pansa. One of the first members of the gens Vibia to achieve political success, he was a Novus homo who rose through the cursus honorum as a result of his friendship with Julius Caesar, under whom he served in Gaul. Originally of Etruscan descent and hailing from Perusia (modern Perugia), and possibly from a family which had been proscribed under Lucius Cornelius Sulla, Pansa was elected Plebeian Tribune in 51 BC where he vetoed a number of anti-Caesarean resolutions of the Senate. During the civil war between Julius Caesar and Pompey, he actively supported the cause of the Caesareans. In 48 BC it is believed he was elected either as an aedile or as a praetor.
In 47 BC Pansa was appointed governor of Bithynia et Pontus, and returned to Rome sometime during 46 BC. In that same year, Caesar appointed Pansa as governor of Cisalpine Gaul to replace Marcus Junius Brutus, a post he took up on 15 March 45 BC. Around this time, he was also elected to the post of augur, one of the priests of Ancient Rome. In early 44 BC, Caesar designated him as the consul for the upcoming year (43 BC) and sometime before 21 April 44 BC, Pansa had returned from Cisalpine Gaul, and was based at Campania, waiting for the situation at Rome to settle down after the assassination of Julius Caesar on 15 March 44 BC.
Recognised as a moderate man and a supporter of peaceful compromise, upon his return to Rome, Pansa became the leader of the moderate Caesareans and one of the leading proponents for the return of the Republic, which put him on a collision course with Marcus Antonius, whom Pansa began to oppose by late 44 BC. He had also begun entering into discussions with Octavianus, Julius Caesar’s adopted son, who was also in Campania at the same time as Pansa. Nevertheless, Pansa was not totally hostile to Marcus Antonius, and while he wanted to limit Antonius’s power, he did not want to destroy him totally, nor was he willing to embrace the anti-Caesarean faction in the Senate and begin a new round of civil wars. Added to this was the fact that Pansa was married to Fufia, the daughter of Quintus Fufius Calenus, who was a key supporter of Antonius.