Early career
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 the actions of Gaius Marius forcibly recalled his father. In 100 BC, after his father was banished due to the political maneuverings of Gaius Marius and Lucius Appuleius Saturninus, Metellus Pius launched a campaign to bring his father 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.
Sometime during the 90s BC, Metellus Pius was elected to the College of Pontiffs as a result of his family's eminence and influence. The outbreak of the Social War saw him employed as a legate in late 89 BC, probably serving the consul Pompeius Strabo in the northern theatre; he won some battles against the Marsi. As a result of these victories, he was elected praetor in the following year (88 BC). During his praetorship, he was tasked with enrolling the Italian allies as new Roman citizens within sixty days, in accordance with the lex Plautia Papiria. Once this was completed, Metellus Pius returned to the front, and replaced the legate Gaius Cosconius on the southern front. He harassed the territory around Apulia, captured the town of Venusia, and defeated the rebel leader, Quintus Poppaedius Silo, who died during the storming of Venusia. Cicero, at the time a young man, remembered hearing Metellus speak at contiones in Rome during this period, most likely during Metellus' praetorship. Cicero remarked of Metellus' ability: 'although no real orator, he was nonetheless not without some capacity for public speech'.
In 88 BC, after being sidelined by his political opponents, Sulla marched his legions on Rome and took the capital. He took revenge on his enemies and forced Marius into exile. Sulla then left Italy and went east to fight in the First Mithridatic War against Mithridates VI of Pontus. In 87 BC, Metellus Pius' command was extended, with his appointment as propraetor, responsible for continuing the war against Samnium. Later that year, however, saw a dispute between the two consuls Lucius Cornelius Cinna and Gnaeus Octavius flare up into war. Cinna, expelled from Rome, met up with the exiled Gaius Marius, and with their ally Quintus Sertorius they marched on Rome and laid siege to the city. During the early phase of this conflict, the Senate, fearing that they may need additional troops and commanders, ordered Metellus Pius to negotiate a peace with the Samnites.
Marching to Rome, he made camp at the Alban Hills, accompanied by Publius Licinius Crassus. Here he met up with Gnaeus Octavius, who had abandoned Rome, but both men soon fell out with each other, over Metellus Pius' troops demanding that their commander take over overall command from Gnaeus Octavius. The Senate then asked him to negotiate with Cinna on their behalf, during which time he recognized Cinna as the legitimate consul. However, with Cinna's occupation of Rome and the executions initiated by Gaius Marius, Metellus Pius decided to abandon Rome and head to North Africa.