The chronology of the early stages of Petreius’ career is unclear. He was in any case the first in his family line to enter into the Senate. Sallust describes him as a military man, who in 62 BC already had a thirty-year-long career in the army as Military tribune, Prefect and Legate behind him. Petreius served at the latest in 64 BC as Praetor, although the exact year he took on this position is unknown.
Petreius served as Legate under the Consul Gaius Antonius Hybrida in 63/62 BC. He led the Senatorial forces in the victory over the revolutionary Lucius Sergius Catilina at Pistoria in early 62 BC, while Hybrida remained away from the battle with a foot ache. During Gaius Julius Caesar’s Consulship of 59 BC, Marcus Petreius allied himself with Caesar’s bitter opponent Marcus Porcius Cato (the Younger).
From 55 BC, Petreius and Lucius Afranius administered the Spanish provinces as Legates, while the official governor Gnaeus Pompeius Magnus remained in Rome. After the outbreak of the Civil War in 49 BC, Petreius and Afranius marched against Caesar, who for his part wished to secure Spain before moving against Pompey in Greece. The two Legates suffered defeat after initial successes, and were forced to capitulate and disband their army on August 2 at Ilerda. Caesar allowed Petreius and Afranius their freedom, and the two traveled to Greece to join Pompey’s forces. After Pompey’s defeat in the Battle of Pharsalus, Petreius and Cato fled from the Peloponnese to North Africa, where the former continued to serve as Legate in the resistance to Caesar. Together with Titus Labienus, Petreius again achieved several successes against Caesar. After the defeat of the Pompeians at Thapsus, Petreius fled with the Numidian King, Juba. As they realized the hopelessness of their situation, Petreius and Juba resolved to take their lives on an estate near Zama: Petreius and Juba decided upon a duel, in which Petreius killed Juba. Petreius then took his own life with the help of a slave.
- Marcus Petreius' Father may have been the Centurion Gnaeus Petreius Atinas, who in 102 BC saved a Legion from destruction by the Cimbri and thereby received the Grass Crown (Pliny, naturalis historia 22, 11).
- Sallust, Bellum Catilinae 59, 6: Homo militaris, quod amplius annos triginta tribunus aut praefectus aut legatus aut praetor cum magna gloria in exercitu fuerat.
- Sallust, Bellum Catilinae 59, 4–61.
- Cassius Dio 38, 3, 2.
- Velleius Paterculus 2, 48, 1.
- Caesar, de bello civili 1, 38–87.
- Cassius Dio 43, 13, 3.
- Appian, Civil Wars 2,95.
- Pseudo Caesar, de bello Africo 91, 1.
- Appian, Civil Wars 2, 100; Seneca, de providentia 2, 10; Pseudo Caesar, de bello Africo 94 switches the roles of Petreius and Juba