Under the latter's patronage he held important state offices and military commands, serving as governor of Syria and leading the expedition to install Herod as king of Judea.

[2] Sosius served as Antony's quaestor (treasurer) sometime between 41 and 39 BC,[3] and in that capacity was stationed at a Roman naval base and mint in Zacynthus, one of the Ionian islands west of the Peloponnese.

[6] In this new capacity, he subdued the intransigent island city of Aradus in northern Phoenicia, and, at Antony's command, installed Rome's ally Herod as king of the Jews by besieging the incumbent Antigonus at Jerusalem.

[15] Sosius and his colleague in office, Gnaeus Domitius Ahenobarbus, were both partisans of Antony; they brought from him a despatch seeking the official ratification of land grants in the east for his children with the Egyptian queen Cleopatra, the so-called Donations of Alexandria,[16] as well as a proposal for the triumvirs to resign their dictatorial powers.

[22] Sosius was one of Antony's chief lieutenants in the ensuing war, and one of few known senators of consular rank to remain loyal as the Antonian cause grew increasingly precarious.

[23] In the summer of 31 BC, Sosius, under cover of fog, routed a squadron of Octavian's fleet led by Lucius Tarius Rufus, but was then driven back by enemy reinforcements under Marcus Agrippa, which cost him the victory and the life of his ally, King Tarcondimotus of Cilicia.

Sosius survived the defeat and spent time in hiding, but was eventually detected and brought before Octavian, who pardoned him at the intercession of one of his own admirals at Actium, Lucius Arruntius.

[30] Octavian, later renamed the emperor Augustus, fully rehabilitated Sosius and gave him a place in the imperial regime, appointing him one of the priestly quindecimviri sacris faciundis who supervised the Saecular Games in 17 BC.