Langkah pertama beliau untuk mencapai cita-citanya ialah dengan memasuki pasukan [[tentera berkuda]] Rom pada 134 SM. Dedikasi beliau sebagai seorang tentera di Numantia telah menarik minat tokoh Rom terkenal iaitu [[Scipio Aemilianus]], cucu kepada [[Scipio Africanus]] iaitu orang yang paling berkuasa di Rom pada ketika itu. Oleh yang demikian, Scipio menganugerahnya dengan berbagai pingat dan penghormatan sehinggakan dia memberitahu orangramai bahawa Marius bakal menjadi penggantinya.
bertanding sebagai tribune dan menang. Pada 122 SM beliau menjadi seorang ''quaestor''. Tetapi beliau tewas dalam pertandingan merebut jawatan aedile dan hampir-hampir tewas dalam pertandingan untuk menjadi praetor pada 116 SM. Beliau juga telah didakwa kerana menyuap rasuah dalam pilihan raya sebelumnya. Selepas itu, beliau dihantar ke Sepanyol Luar untuk menentang penjahat ''bandits'' di provinsi tersebut. Sebaik sahaja Marius pulang, beliau membuat pakatan politiknya dengan mengahwini Julia dari keluarga Caesar: isterinya merupakan ibu saudara diktator yang akan datang iaitu [[Julius Caesar]]. Perkahwinan ini telah mengukuhkan pengaruhnya di arena politik Rom.
He returned to army service during the Jugurthine war under Caecilius Metellus, where he did all he could to upstage his commander and take advantage of all the opportunities for glory that were presented to him. He also made sure he was loved by his own men and shared in all their work, meals and accommodations. He made a lot of friends in the army that way and his name became famous in Libya then Rome. His soldiers even wrote home stating that he should be elected consul to end the war they were involved in and Metellus was not too pleased with this and eventually became hostile to Marius.[15] Marius knew the Metelli and the rest of the nobles were set against his obtaining the consulate and he resented that and wanted to win despite their lack of support towards him for the attainment of that magistrature.[16] Marius then sailed back to Rome and asked for the consulship after attacking Metellus in the Senate and promising that he would return with Jugurtha alive or kill the Numidian leader.[17]
