Marcus Marius was the younger brother of the Gaius Marius who was consul seven times.

Marcus was a few years younger than Gaius Marius, hailing from the same relatively wealthy equestrian family.^[1] During his brother's series of successive consulships between 104 and 100 BC, Marcus was elected praetor, probably for 102 BC. He served his praetorship and a following year as proconsul in Spain, probably Hispania Ulterior like his brother a decade before him.^[2] During his time there, he campaigned against Lusitanian tribes with the support of the Celtiberians and founded a city.^[3] It is also possible that his term in Spain was not prorogued if he was instead elected for 101 BC, though this is less likely.^[4] His elder brother may have manipulated the sortition of the highly sought-after province in anticipation of a possible consular campaign. Marcus, however, never attained the consulship, as he likely died in the 90s BC.^[5]

Marcus adopted a member of the Gratidii later named Marcus Marius Gratidianus. The Gratidii were a wealthy and influential family in the city of Arpinum, from which both they, the Marii, and the Tullii Cicerones hailed.^[6] Gratidianus was elected praetor twice in the 80s BC (most probably 85 and 82) and was killed during Sulla's civil war shortly after the Battle of the Colline Gate.^[7]