Born to a recently distinguished senatorial family, he became one of the most violent and bloodthirsty partisans of the consul Cornelius Cinna and his ally, Gaius Marius, in the civil war of 87 BC.

During the conflict, Fimbria served them as both cavalry commander and negotiator, and took a leading part in the political purges that followed their capture of Rome, putting a number of prominent aristocrats to death.

In Asia, after having defeated Mithridates, Fimbria was confronted by Lucius Cornelius Sulla, the first Roman general in charge of the war, who had been removed from his command by Cinna and Marius, but entered into a rebellion against the official government in Rome.

[7] When Octavius was finally forced to capitulate, and allow his rivals into Rome,[8] Fimbria participated in the subsequent purge of Marius' and Cinna's enemies in the city.

When asked what charges could he possibly bring against such a well-reputed man, Fimbria declared that the victim had failed to submit his body to the full thrust of the blade.

[16] The expedition was to be led by Marius's replacement consul, Lucius Valerius Flaccus, and Fimbria, owing presumably to his position as quaestor,[ii] joined him as the foremost member of his staff.

The expedition began badly, for after sailing from Brundisium several ships were lost to a storm at sea, and, after landing in Greece, an advance party was bloodied by Mithridatic forces nearby.

[22] Fimbria, encharged with the cavalry, led the advance guard of the army a few days ahead of the main force, and began to stir up trouble by allowing his troops to plunder the surrounding area as they marched through Macedonia and Thrace.

[25] Fimbria, around this time, entered into a dispute with another officer over lodgings, and Flaccus showed little sympathy for his recalcitrant subordinate, deciding in favor of the latter party.

But in 85 Sulla crossed over from Greece to Asia, made peace with Mithridates, and turned his arms against Fimbria, who, seeing that there was no chance of escape, committed suicide, with the assistance of a slave.