Barely left out of teenagehood, he was made Pontifex Maximus, which office he later held again in 81 BC, Quaestor in 97 BC, Tribune in 92 BC, and Consul in 80 BC, having been a Praetor in 89 BC . He withdrew from Rome during the civil fights between Marius and Sulla, returning afterwards with the latter.
As a military figure, he had his first commands in the war in 109-107 BC, when he accompanied his father Quintus Caecilius Metellus Numidicus in Numidia during Jugurthine War, where his father took him as a simple cadet. . He later revealed to be one of the best subordinates of Sulla, but without any other objective than the one of fighting the demagogy with which Marius threatened Rome, never participating in the atrocious violence which marked such a troubled phase of the History of Rome. He was one of the Roman commanders in the Social War in 88 BC. A Proconsul at that time, Pius stormed Venusia and killed the leader of Marsi Quintus Poppaedius Silo . In 87 BC Pius by the order of Senate tried to make peace with the Samnites.
Made a Consul in 80 BC, he was then sent to Spain to combat Quintus Sertorius, where Sertorius and Marcus Perpenna Vento faced the Roman power, establishing his bases in Metellinum (today Medellín) Castra Caecilia (today Cáceres), Viccus Caecilius, at the Sierra de Gredos, and at Caeciliana, near Setúbal. After eight years of resistance, the rebels were forced to cede to the military ability of Metellus Pius, who had as a companion, in the final phase of the war, the valuable Gnaeus Pompeius Magnus, with whom, despite his greater merit, he shared the glories of the Triumph after the death of Sertorius, marked in the end of 71 BC .
He married Licinia Crassa Secunda or Minor, daughter of Lucius Licinius Crassus Orator, and wife Mucia Secunda, from whom he had no children; for this reason he adopted his nephew by marriage and son of his second cousin Publius Cornelius Scipio, renamed Quintus Caecilius Metellus Pius Scipio Nasica, son of his sister in law's marriage Licinia Crassa Prima or Major to Publius Cornelius Scipio Nasica Serapio, in turn a son of Publius Cornelius Scipio Nasica Serapio and Caecilia Metella, daughter of Quintus Caecilius Metellus Macedonicus.