Pansa was the son of moneyer Gaius Vibius 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.