Decimus Brutus spent his youth mainly in the company of Publius Claudius and Mark Antony. His mother was Sempronia Tuditani, wife of Decimus Junius Brutus who was consul in 77 B.C.E. He was adopted by Aulus Postumius Albinus, but kept his own family name, only adding his adoptive father's cognomen Albinus.
When the Republican Civil War broke out, Decimus Brutus sided with his commander, Caesar, and was entrusted once again with fleet operations.
The Greek city of Massilia (present-day Marseille) sided with Pompey the Great and Caesar, in a hurry to reach Spain and cut Pompey off from his legions, left Decimus Brutus in charge of the naval blockade of Massilia. Within thirty days, Decimus Brutus built a fleet from scratch and secured the capitulation of Massilia.
On the Ides of March (March 15), when Caesar decided not to attend the Senate meeting due to the concerns of his wife, Calpurnia, Decimus Brutus persuaded him to go, dismissing Calpurnia's concerns. When Caesar arrived in Pompey's theatre for the Roman Senate meeting, Decimus and the rest of the conspirators attacked and assassinated him. According to Nicolaus of Damascus, Decimus Brutus was the third to strike Caesar, stabbing him in the side.
In 43 BCE Decimus Brutus occupied Mutina, laying in provisions for a protracted siege. Antony obliged him, and blockaded Decimus Brutus' forces, intent on starving them out.
Nevertheless, the consuls of the year, Aulus Hirtius and Gaius Vibius Pansa Caetronianus, marched northward to raise the siege. Guided by Cicero, the Senate was inclined to view Mark Antony as an enemy. Octavian, the nineteen-year-old heir of Caesar, and already raised to the rank of Propraetor, accompanied Gaius Pansa north. The first confrontation occurred on April 14 at the battle of Forum Gallorum, where Antony hoped to deal with his opponents piecemeal. Antony defeated the forces of Gaius Pansa and Octavian, which resulted in Pansa suffering mortal wounds; however, Antony was then defeated by a surprise attack from Hirtius. A second battle on 21 April at Mutina resulted in a further defeat for Antony and Hirtius' death. Antony withdrew, unwilling to become the subject of a double circumvallation as Caesar had done to Vercingetorix at Alesia.
With the siege raised, Decimus Brutus cautiously thanked Octavian, now commander of the legions that had rescued him, from the other side of the river. Octavian coldly indicated he had come to oppose Antony, not aid Caesar's murderers. Decimus Brutus was given the command to wage war against Antony, but many of his soldiers deserted to Octavian. His position deteriorating by the day, Decimus Brutus fled, attempting to reach Macedonia, where Marcus Junius Brutus and Gaius Cassius Longinus had stationed themselves. He left Italy, abandoning his legions, but was executed en route by a Gallic chief loyal to Mark Antony, becoming the first of Caesar's assassins to be killed.
Several letters written by Decimus Brutus during the last two years of his life are preserved among Cicero's collected correspondence.
In the 1993 Allan Massie book entitled Caesar Decimus Junius Brutus Albinus narrates his story and reason for joining in Caesar's assassination while being held captive by the Gallic chief.