Metellus Pius, a member of the distinguished plebeian "gens Caecilia", was the son of Quintus Caecilius Metellus Numidicus, who was consul in 109 BC. His career began in that same year, when he accompanied his father to Numidia as his "contubernalis" (cadet) during the Jugurthine War, returning to Rome in 107 BC, when his father was forcibly recalled by the actions of Gaius Marius. In 100 BC, after his father was banished as a result of the political manoeuvrings of Gaius Marius and Lucius Appuleius Saturninus, Metellus Pius launched a campaign to have his father brought back from exile. He produced a petition in 99 BC to this effect, and his constant pleading on the subject resulted in Quintus Calidius, the Plebeian Tribune of 98 BC passing a law which allowed his father to return. As a result of his fidelity, he was given the agnomen “Pius” for the constancy and inflexibility with which he fought for his father's political rehabilitation and return to Rome.