[1] Born c. 92 BC,[2] Trebonius' father was an eques,[3] but had not been a magistrate, and the son was considered a novus homo ("new man"), one of several in Caesar's circle.
[4] He served as quaestor around 60 BC,[5] during which he attempted to prevent the adoption of Publius Clodius Pulcher into a plebeian family, against the wishes of the triumvirs.
Cato, a noted opponent of Pompey, spoke against the bill, attempting to filibuster the motion, causing Trebonius to firstly expel him from the Forum, before ordering him to be taken to prison.
[7] Eventually the law was passed, with the five-year commands to Pompey, who received the two Spanish provinces, and Marcus Licinius Crassus, who obtained Syria.
In 54 BC he accompanied Caesar during his second expedition to Britain, where he was placed in charge of three legions who successfully defeated a concerted attack by the forces of Cassivellaunus.
[10] From here he accompanied Caesar in coming to the aid of Quintus Cicero who was besieged during the beginning of Ambiorix's revolt against Roman control of Gaul.
[19] Before he commenced the siege, Trebonius collected labourers and cattle from the Province, ordered timber and wood suitable for wattle-work to be brought up, and then proceeded to construct the principal terrace.
[22] This led to his soldiers becoming disgruntled, as they were hoping to sack the town and they blamed Trebonius for halting the attack, and they were only kept under control with great difficulty.
[23] Then in late August, Trebonius was surprised when the Massiliots burst out of the town, took advantage of the lack of guards posted around, and destroyed the siege equipment which had breached the walls.
[30] At the end of the year he was given a proconsular command and sent to govern Hispania Ulterior, replacing Quintus Cassius Longinus, who was accused of mismanaging the province.