Marius was born in 157 BC in the town of Arpinum in southern Latium. The town had been conquered by the Romans in the late fourth century BC and was given Roman citizenship without voting rights. Only in 188 BC did the town receive full citizenship. Although Plutarch claims that Marius' father was a laborer, this is almost certainly false since Marius had connections with the nobility in Rome, he ran for local office in Arpinum, and he had marriage relations with the local nobility in Arpinum, which all combine to indicate that he was born into a locally important family of equestrian status. The problems he faced in his early career in Rome show the difficulties that faced a "new man" (novus homo).

There is a legend that Marius, as a teenager, found an eagle's nest with seven chicks in it – eagle clutches hardly ever have more than 3 eggs; even if two females used the same nest, finding 7 offspring in a single nest would be exceptionally rare. Since eagles were considered sacred animals of Jupiter, the supreme god of the Romans, it was later seen as an omen predicting his election to the consulship seven times. Later, as consul, he decreed that the eagle would be the symbol of the Senate and People of Rome.

In 134 BC, he was serving with the army at Numantia and his good services brought him to the attention of Publius Cornelius Scipio Aemilianus Africanus. Whether he arrived with Scipio Aemilianus or was already serving in the demoralized army that Scipio Aemilianus took over at Numantia is not clear. It would seem that even at this early stage in his army career, Marius had ambitions for a political career in Rome. He ran for election as one of the twenty-four special military tribunes of the first four legions who were elected (the rest were appointed by the magistrate who raised the legion). Sallust tells us that he was unknown by sight to the electors but was returned by all the tribes on the basis of his accomplishments.

Next, he ran for the quaestorship after losing an election for local office in Arpinum. The military tribunate shows that he was already interested in Roman politics before the quaestorship. Perhaps he simply ran for local office as a means of gaining support back home, and lost to some other local worthy. Nothing is known of his actions while quaestor.

In 120 BC, Marius was returned as plebian tribune for the following year. He won with the support of Quintus Caecilius Metellus (later known as Metellus Numidicus), who was an inherited patronus. The Metelli, though neither ancient nor patrician, were one of the most powerful families in Rome at this time. During his tribunate, Marius pursued a populares line. He passed a law that restricted the interference of the wealthy in elections. In the 130s voting by ballot had been introduced in elections for choosing magistrates, passing laws and deciding legal cases, replacing the earlier system of oral voting. The wealthy continued to try to influence the voting by inspecting ballots and Marius passed a law narrowing the passages down which voters passed to cast their votes in order to prevent outsiders from harassing the electors. In the passage of this law, Marius alienated the Metelli, who opposed it.

Soon thereafter, Marius ran for the aedileship and lost. This loss was at least in part due to the enmity of the Metelli. In 116 BC he barely won election as praetor for the following year (presumably coming in sixth) and was promptly accused of ambitus (electoral corruption). He barely won acquittal on this charge, and spent an uneventful year as praetor in Rome (as Urban Praetor, Peregrine Praetor or President of the extortion court). In 114 BC, Marius' imperium was prorogued and he was sent to govern Lusitania, where he engaged in some sort of minor military operation. During this period in Roman history governors seem regularly to have served two years in Hispania, so he was probably replaced in 113 BC.

He received no triumph on his return and did not apparently run for the consulship, but he did marry Julia, the aunt of Julius Caesar. The Julii Caesares were a patrician family, but at this period seem to have found it hard to advance above the praetorship. (Only once in the second century – in 157 BC – did a member of the family become consul.) To judge by this marriage, Marius had apparently achieved some substantial political or financial influence by this point (possibly from his governship in Hispania).

The Marii were the inherited clients of the Caecilii Metelli and a Caecilius Metellus had aided Marius' campaign for the tribunate. Although he seems to have had a break with the Metelli as a result of the laws he passed while tribune, the rupture was not permanent, since in 109 BC Quintus Caecilius Metellus took Marius with him as his legate on his campaign against Jugurtha. Legates (legati) were originally simply envoys sent by the Senate, but men appointed as legates by the Senate were used by generals as subordinate commanders, usually becoming the general's most trusted lieutenant. Hence, Metellus had to have asked the Senate to appoint Marius as legate to allow him to serve as Metellus' subordinate. In Sallust's long account of Metellus' campaign no other legates are mentioned, so it is assumed that Marius was Metellus's senior subordinate and right-hand man. Thus Metellus was using Marius' military experience, while Marius was strengthening his position to run for the consulship. The rupture in 119 BC may have been exaggerated after the fact in light of his later and much more serious disagreement with Metellus about Numidia.

By 108 BC, Marius conceived the desire to run for the consulship. Despite lack of approval from Metellus (brought on by Marius' status as a novus homo) who instead advised Marius to wait and run with Metellus' son (who was only twenty, which would signify a campaign 20 years in the future) Marius began to campaign for the consulship. Sallust claims that this was catalyzed, in part, by a fortune-teller who "told him that great and wonderful things were presaged to him that he might therefore pursue whatever designs he had formed trusting to the gods for success, and that he might try fortune as often as he pleased for that all his undertakings would prosper." Marius soon earned the respect of the troops by his conduct towards them, eating his meals with them and proving he was not afraid to share in any of their labours. He also won over the Italian traders by claiming that he could capture Jugurtha in a few days with half Metellus' troops. Both groups wrote home in praise of him, suggesting that he could end the war quickly unlike Metellus, who was pursuing a policy of methodically subduing the countryside. Eventually Metellus gave in, realizing that it was counterproductive to have a resentful subordinate.

Under these circumstances, Marius was triumphantly elected consul later that year, for 107 BC. He was campaigning against Metellus's apparent lack of swift action against Jugurtha. Because of the repeated military debacles from 113 BC to 109 BC and the accusations that the oligarchy was open to flagrant bribery, it became easier for the virtuous new man who had worked with difficulty up the ladder of offices was elected as an alternative to the inept or corrupt nobility. The Senate had a trick up its sleeve, however. In accordance with the provisions of the Lex Sempronia on Consular provinces, which dictated that the Senate in a given year was to determine the Consular provinces for the next year at the end of year before the elections, the Senate decided not to make the war against Jugurtha one of the provinces and to prorogue Metellus in Numidia. Marius got around this through a ploy that had been used in 131 BC. In that year there was a dispute as to who should command the war against Aristonicus in Asia, and a tribune had passed a law authorizing an election to select the commander (there was precedent for this procedure from the Second Punic War). A similar law was passed in 108 BC and Marius was voted the command by the People in this special election. Metellus shed bitter tears when he learned of the decision. Upon returning home, he avoided meeting Marius, and was granted a Triumph and the agnomen Numidicus (conqueror of Numidia).