Marius was born in Arpinum, a town in Latium, to an equestrian family. As a soldier, he distinguished himself in both Spain and Africa. When the Romans were not fairing well against Jugurtha, Marius returned to Rome to run for consulship, promising the people a victory. He was not a member of one of the noble families, but was nevertheless elected to be consul in 107 BC. As a man from a family that had never held the consulship before, Marius was considered a novus homo, or "new man", of which there were few during the Republic.
After winning the election, Marius had the plebeian assemblies grant him a generalship. He then recruited a new army without consulting the senate and returned to Numidia to deal with Jugurtha. Marius' army performed very well and forced Jugurtha to flee to a neighboring country. Unable to capture Jugurtha, negotiations were begun by a young officer named Cornelius Sulla. Sulla did well, and Jugurtha was handed over to the Romans. Marius took him back to Rome, where a triumph was held in 104 BC.
Marius had reformed the Roman army, by recruiting men from the urban poor. Traditionally, the Roman army had consisted of farmer-soldiers. They were expected to provide their own equipment, and only fought as long as needed. This new army was far better trained, equipped more uniformly, and promised land upon their discharge.
The effectiveness of this army would soon be demonstrated again. In several battles, Marius saved Italy from invasion by two German tribes, the Teutones and Cimbri.
During this time, Marius held consulship every year But, Marius began to lose support due to his political friendship with the tribune Saturninus, who often times served as his hatchet man. Eventually Marius was forced to disassociate himself from bloody Saturninus. In 99 BC, the Senate declared a senatus consultum against Saturninus, and Marius was forced to have his friend killed. Soon after, Marius retired to a villa in Pergamum. Sulla then rose to take his place as the best Roman general during the ensuing Social War.
When Mithridates, the king of Pontus, invaded Pergamum, Marius was called out of retirement to lead an army against him. Enraged, Sulla and his army seized Rome in 88 BC. Marius and his allies, who had survived the attack, were forced to flee. When Sulla headed east, Marius returned to Rome and declared himself consul for the seventh time. After a bloody few months, Marius died at the age of 70.