Marcus Junius D. f. D. n. Silanus was a member of the Junii Silani, a noble Roman family, who held the consulship in 109 BC.
Because there are only few and short sources about the history of the Roman Republic in the second half of the second century BC, we have to rely on suppositions as to which public offices Silanus held before his consulate. In 145 BC he was perhaps the director of a mint. He is probably identical with the tribune of the people Marcus Junius D. f., who introduced in 124 or 123 BC a law against exploitative Roman governors (lex Iunia), which preceded the lex Acilia repetundarum of the tribune Manius Acilius Glabrio (123 or 122 BC). In 113 or 112 BC Silanus was perhaps praetor in Spain.
In 109 BC Silanus achieved to become consul as the first member of his family, the Junii Silani. He held this highest public office together with Quintus Caecilius Metellus Numidicus, who had to continue the war against Jugurtha, king of Numidia, whereas Silanus undertook to fight against the Cimbri. To increase the power of Rome Silanus abolished the exemptions from the military service. Probably before their battle with the consul the traveling Cimbri had asked to be given a domicile on Roman territory, but the Senate had declined their request. Then Silanus rushed towards these people but he was defeated on a not exactly localized place in Gallia Transalpina.
In 104 BC the tribune of the people Gnaeus Domitius Ahenobarbus accused Silanus of his military failure, but the former consul was acquitted.