He is first mentioned for crushing the revolt of the town of Fregellae in 125 BC. He was elected consul in 121 BC with Q. Fabius Maximus Allobrigicus, and while Fabius was campaigning in Gaul, he took part in perhaps the most decisive event of Roman history to that point.
When Gaius Gracchus and M. Fulvius Flaccus were defeated for re-election by Opimius and Fabius, Gracchus organized a mass protest on the Aventine Hill. Alarmed by this action, the Senate passed the motion senatus consultum ultimum, which Opimius understood as an order to suppress their activites by any means necessary -- including force. He gathered an armed force of Senators and their supporters, and confronted Gracchus and his followers, an act which quickly became a pitched battle inside the city of Rome. Gracchus, Flaccus, and many of their followers were slain in this conflict, and after clearing the streets of his opponents, Opimius established a quaestio or tribunal that condemned to death 3000 people accused of being supporting Gracchus.
Opimius was prosecuted for these violent actions in 120 BC, but Carbo won his acquittal. Opimius' victory established the senatus consultum ultimum in Roman constitutional practice, providing a limitless tool that the various Roman factions used against each other in the following years as the Republic slipped increasingly into violence and civil war.
In 116 BC, Opimius headed a commission that divided Numidia between Jugurtha and his brother Adherbal. Suspecting that the commission had been influenced by bribes from Jugurtha, its members were later investigated by a tribunal that censured their conduct. Opimius was forced into exile, where he later died.