He was born in Spain, perhaps of Italian origin. In 49 BC he fought as a supporter of Julius Caesar in Spain against an army allied with Pompey. In 44 BC he was tribune of the people and after the assassination of Caesar he was allied to Mark Antony. At the beginning of 43 BC Antony was besieged in Mutina and was supported by Decidius Saxa. In 42 BC, after the founding of the second Triumvirate, Saxa was, together with Gaius Norbanus Flaccus, appointed by Mark Antony to lead the advance force of eight legions into Thrace before the Battle of Philippi.
Saxa later went on to be appointed governor of Syria by Antony (41 BC) while Norbanus was elected consul in 38 BC, recognizing the great prestige of the victory over the liberatores. He was heavily defeated near Antioch, when Quintus Labienus led a Parthian intervention in Syria in 40 BC. He fled to Cilicia where he was captured and executed by the Parthians. His legions reportedly suffered heavy defeats and several of his aquilae were seized, being returned to Rome first after a brief Roman war against Parthia and negotiations by then Roman emperor Augustus in 20 BC. The eagle standards were returned together with those captured in 53 BC from Marcus Licinius Crassus, a great propaganda victory for Rome.
He is also mentioned as the first Gromatici.
- Ronald Syme: Who was Decidius Saxa? In: Journal of Roman studies 27 (1937), p. 127–137 = the same: Roman Papers, vol. 1, Oxford 1979, p. 31–41.
- Cicero, Philippica 11.12; 13.27; the Italian origin is a suggestion of Ronald Syme (see references).
- Julius Caesar, Commentarii de Bello Gallico 1.66.3
- David L. Vagi (2000). Coinage and History of the Roman Empire, C. 82 B.C.--A.D. 480: History. Taylor & Francis. pp. 71–. ISBN 978-1-57958-316-3.
- Cicero, Philippica 11.12; 13.27.
- Cassius Dio, Roman History 47.35-36
- Cassius Dio, Roman History 48.25; Marcus Velleius Paterculus, Roman History 2.78.1
- Cassius Dio, Roman History 48.25.2