Gaius Manilius had at the beginning of his year of office as tribune (Dec. 67 BCE) successfully gained passage of a law (de libertinorum suffragiis) giving freedmen the privilege of voting together with those who had manumitted them, that is, in the same tribe as their patroni; this law, however, is almost immediately declared null and void by the senate.

Both parties in the state are offended by the law, and Manilius endeavors to secure the support of Pompey by proposing to confer upon him the command of the war against Mithridates VI of Pontus with full powers to make war and peace and to organize the whole Roman East.

Pompey is still in the East in 66, resettling pirates as peaceful farmers, when Manilius, carries through, against weakened opposition, a bill appointing Pompey to the command against Mithridates, with unlimited power.

The proposal is supported by Cicero, who holds the praetorship in 66, in his speech, Pro lege Manilia, and carries almost unanimously.

Manilius is later accused by the aristocratic party on some unknown charge and defended by Cicero.

He was probably convicted, but nothing further is heard of him.