He was a legate in Julius Caesar army during the Gallic wars, but when the dictator's returned to Rome after the final defeat of the Republican faction in the battle of Munda (45 BC), Decimus Brutus joined the group of conspirers that were planning his death. Apparently, Caesar continued to trust in him and, since he was a relative, mentioned him in the will he made in October 45. Moreover, in 44 BC, he was elected praetor by the centuriate assembly, by personal appointment of Caesar. Decimus Brutus was chosen to be praetor peregrinus and destined to be the governor of Italian Gaul in the following year.
In the Ides of March (March 15), the day of Caesar assassination, Decimus Brutus accompanied the dictator in his morning appointments, previous to the senate meeting. The purpose was to be alert to send a message to rest of the conspirers, waiting in the Forum. When Caesar arrived in Pompey's theatre, where the House was to meet, he was killed by the group, with Decimus among them.
The assassins received an amnesty in the next day, issued by the senate with the instigation of Marcus Antonius, Caesar's partner in consulship. But the situation was not peaceful: Rome's population and the legionaries of Caesar's legions wanted to see the conspirers punished. The group decided to lay low and Decimus used his office of praetor peregrinus to stay away from Rome. The climate of reconciliation was over and slowly the conspirers were starting to feel the strain of the assassination. Thus, in the beginning of 43 BC, Decimus hurried to his province and started to levy his own troops. He was ordered by the senate to surrender his province to Antonius, but denied. This was an act of war that Antonius was only happy to oblige. With his own political situation on the verge of disaster, and himself declared public enemy, defeating Decimus was a way to regain his ascent and get control of the powerful Italian Gaul. The first confrontation occurred in April 14, in the battle of Forum Gallorum, where Antonius' troops besieged Decimus' army in Mutina. Antonius was defeated by the forces of consul Pansa (aided by Octavianus, then only 19 years old but already a propraetor). A second battle a few days later (Mutina on April 21) returned the same result: Antonius defeated without managing to catch Decimus and a dead consul. Despite this luck, Decimus Brutus decided to flee to Macedonia, where Marcus Junius Brutus and Cassius were garrissoned. He left Italy, abandoning his legions, but was killed shortly afterwards, before reaching his fellow assassins.