He was known to have been a legate of Caesar's since about 54 BC, and served as an envoy to Pompey in 50. During the Roman Civil Wars he served in Spain, he might have been a tribune in 48, and in 47 was at Antioch. He was a praetor in 46 and governor of Transalpine Gaul in 45. Caesar had nominated Hirtius and Pansa for the consulship for 43; after the assassination, Hirtius was deeply involved in the maneuvering between parties.
Initially a supporter of Mark Antony, Hirtius was successfully lobbied by Cicero, who was a personal friend, switched his allegiance to the senatorial party, and set out with an army to attack Antony, who was besieging Mutina. In concert with Octavian, Hirtius compelled Antony to retire, but in the fighting Hirtius was slain (around 25 April or 27 April). He was honored with a public funeral, along with Gaius Vibius Pansa Caetronianus, who died a few days later.
Hirtius added an eighth book to Caesar's De Bello Gallico, and is the likely author of De Bello Alexandrino. The ancients thought he also wrote the De Bello Africo and De Bello Hispaniensis, but it is now considered more likely that he acted as an editor. Hirtius' correspondence with Cicero was published in nine books, but has not survived.
Suetonius in Chapter 68 of his Life of Augustus writes that Lucius Antonius, the brother of Mark Antony accused the Emperor Augustus for having "given himself to Aulus Hirtius in Spain for three hundred thousand sesterces." This alleged homosexual liaison must have taken place in 46 BC during the civil wars when Julius Caesar took Augustus to Spain and Aulus Hirtius was serving there. At the time the future Emperor Augustus was 19 years old.