Brutus was the son of Marcus Junius Brutus the Elder and Servilia Caepionis. His father was a legatus to Pompey the Great; his mother was the half-sister of Cato the Younger, and later became Julius Caesar's mistress. Some sources refer to the possibility of Caesar being his real father, but this is unlikely since Caesar was 15 at the time of Brutus' birth. Brutus' uncle, Quintus Servilius Caepio, adopted him when he was a young man and Brutus was known as Quintus Servilius Caepio Brutus for an unknown period of time.
Brutus held his uncle in high regard and his political career started when he became an assistant to Cato, during his governorship of Cyprus. During this time, he enriched himself by lending money at high rates of interest. He returned to Rome a rich man, where he married Claudia Pulchra. From his first appearance in the Senate, Brutus aligned with the Optimates (the conservative faction) against the First Triumvirate of Marcus Licinius Crassus, Gnaeus Pompeius Magnus and Gaius Julius Caesar.
When the Roman Civil War broke out in 49 BC between Pompey and Caesar, Brutus followed his old enemy and present leader of the Optimates, Pompey. When the Battle of Pharsalus began, Caesar ordered his officers to take him prisoner if he gave himself up voluntarily, and if he persisted in fighting against capture, to let him alone and do him no violence. After the disaster of the battle of Pharsalus, Brutus wrote to Caesar with apologies and Caesar immediately forgave him. In his letter Brutus declared he was a strong supporter of democracy and continually pushed it throughout the letter. Caesar accepted him into his inner circle and made him governor of Gaul when he left for Africa in pursuit of Cato and Metellus Scipio. In 45 BC, Caesar nominated Brutus to serve as urban praetor for the following year.
Also, in June 45 BC, Brutus divorced his wife and married his first cousin, Porcia Catonis, Cato's daughter. According to Cicero the marriage caused a semi-scandal as Brutus failed to state a valid reason for his divorce from Claudia other than he wished to marry Porcia. The marriage also caused a rift between Brutus and his mother, who resented the affection Brutus had for Porcia.
Around this time, many senators began to fear Caesar's growing power following his appointment as dictator for life. Brutus was pressured into joining the conspiracy against Caesar by the other senators and he also discovered messages written on the busts of his ancestors. Brutus, influenced by his loyalty to Cato and Porcia, finally decided to move against Caesar in 44 BC. His wife was the only woman privy to the plot.
The conspirators planned to carry out their plot on the Ides of March that same year. On that day, Caesar was delayed going to the Senate because his wife, Calpurnia Pisonis, tried to convince him not to go. The conspirators feared the plot had been found out. Brutus persisted, however, waiting for Caesar at the Senate, and allegedly still chose to remain even when a messenger brought him news that would otherwise have caused him to leave. When Caesar finally did come to the Senate, they attacked him. Publius Servilius Casca was allegedly the first to attack Caesar with a blow to the shoulder, which Caesar blocked. However, upon seeing Brutus was with the conspirators, he covered his face with his toga and resigned himself to his fate. The conspirators attacked in such numbers that they even wounded one another. Brutus is said to have been wounded in the hand.
After the assassination, Brutus was approached with a compromise: if Caesar was declared a tyrant, then all of Caesar's acts and senatorial appointments - Brutus' urban praetorship among other offices given to some of the assassins before they killed Caesar - would be declared null and void. This would have meant that Brutus' urban praetorship was illegal and elections would have had to be held. Conversely, if he agreed to recognize Caesar's appointments, he and the other assassins would be granted amnesty and retain their positions. Brutus accepted the offer, and Caesar was not declared a tyrant. Part of the offer was that Brutus had to leave Rome, which he did. After leaving Rome, Brutus lived in Crete from 44 to 42 BC.
In 43 BC, after Octavian received his consulship from the Roman Senate, one of his first actions was to have the people that had assassinated Julius Caesar declared murderers and enemies of the state. Marcus Tullius Cicero, angry at Octavian, wrote a letter to Brutus explaining that the forces of Octavian and Mark Antony were divided. Antony had laid siege to the province of Gaul, where he wanted a governorship. In response to this siege, Octavian rallied his troops and fought a series of battles in which Antony was defeated. Upon hearing that neither Antony nor Octavian had an army big enough to defend Rome, Brutus rallied his troops, which totaled about 17 legions. When Octavian heard that Brutus was on his way to Rome, he made peace with Antony. Their armies, which together totaled about 19 legions, marched to meet Brutus and Gaius Cassius Longinus. The following battles are known as the Battle of Philippi. The First Battle of Philippi was fought on October 3, 42 BC, in which Brutus defeated Octavian's forces, although Cassius was defeated by Antony's forces. The Second Battle of Philippi was fought on October 23, 42 BC and ended in Brutus' defeat.
After the defeat, he fled into the nearby hills with only about four legions. Knowing his army had been defeated and that he would be captured, Brutus committed suicide. Among his last words were, according to Plutarch, "By all means must we fly; not with our feet, however, but with our hands." Antony, as a show of great respect, ordered his body to be wrapped in his own most expensive cloak. Brutus was cremated, and his ashes were sent to his mother, Servilia Caepionis. His wife Portia was reported to have committed suicide upon hearing of her husband's death.