Marcus Petreius was an experienced solder and a supporter of Pompey during the Great Roman Civil War. He was defeated at Ilerda in Spain in the first year of the war, but survived to take part in the defeats at Pharsalus and at Thapsus, committing suicide after the second of these battles.
Petreius is first mentioned in 62 BC, during the campaign against Cataline, the leader of a plot to seize power in Rome. This plot had been unmasked by Cicero and Cataline had fled to Etruria, where he had raised an army. The consul Gaius Antonius led the army sent to defeat Cataline, but on the day of the battle he was ill (or pretending to be ill to avoid an uncomfortable confrontation with Cataline), and command of the army passed to Petreius, who won the final battle of the campaign, near Pistoria.
By 59 BC at the latest Petreius was a Senator. Caesar, who was one of the Consuls for the year, clashed with Cato over his land reform, and threatened to arrest him in the Senate. A number of other senators went to accompany Cato, but it was Petreius who made the most telling contribution, responding to a rebuke from Caesar by saying 'I prefer to be with Cato in his cell rather than here with you'.
In 55 BC Pompey the Great was given the provinces of Spain for his proconsular year, but he had no intention of leaving Spain. Instead Petreius and L. Afranius were sent out to Spain to rule the province in his place. The two men were still in Spain when the Great Roman Civil War broke out in 49 BC. Between then they commanded five legions and a large number of auxiliaries, the most powerful of Pompey's armies. Caesar decided to ignore Pompey in the Balkans and instead head west to deal with the threat from Spain. Petreius and Afranius made their stand at Ilerda, where after some early successes they were forced to surrender. Their army was disbanded and the two commanders were allowed to leave unharmed.
Both men made their way to Pompey. Petreius was present at the battle of Pharsalus (48 BC), the decisive defeat of the Pompeian cause in the Balkans, but escaped from the battle. He joined up with Cato at Patrae, and stayed with him when Cato decided to continue the fight from Africa. He fought at the battle of Ruspina, where he was wounded, and at Thapsus (46 BC), where the Pompeian cause in Africa was finally defeated. In the aftermath of this battle Petreius accompanied King Juba I of Numidia as he attempted to find a safe haven. When this failed the two men decided to commit suicide, fighting a duel to the death in one of Juba's country houses.