The career of Aulus Didius Gallus up to 51 can be partly reconstructed from an inscription from Olympia (CIL III, 7247 + CIL III, 12278). He was quaestor under Tiberius, probably in 19. He served as a legate of the proconsul of Asia, as prefect of cavalry, and as proconsul of Sicily, although the dates of these appointments are unknown. He was curator aquarum (superintendent of aqueducts) from 38 to 49, consul in 39, and a member of the XVviri. He received triumphal regalia as an imperial legate under Claudius, probably in Bosporus: Tacitus records that he commanded forces there that were withdrawn in 49. After this he appears to have taken up another proconsular appointment, possibly in Asia or Africa.
His later career is described by Tacitus. In 52 he was made governor of Britain, following the death in office of Ostorius Scapula, at a time when the situation was deteriorating as a result of a string of rebellions. The south-east was securely held, but despite the defeat of Caratacus the previous year, the tribes of what is now Wales, particularly the Silures, continued to hold out. Venutius' first insurrection against Queen Cartimandua of the Brigantes occurred during Didius' rule and he despatched troops under Caesius Nasica to aid her.
Didius acted to quell the rebels rather than enlarge the empire during his rule, which lasted until AD 57. Although criticised by Tacitus as being reactive and defensive, he was probably acting on instructions from Claudius who did not consider the benefits of further conquest in difficult terrain to be great enough to warrant the risk. Instead, Didius built roads and forts at the borders such as those at Usk to contain the native population. After five years in the post, covering the last two years of the reign of Claudius and the first three of Nero, Didius was replaced by Quintus Veranius.
Quintilian tells us that, after several years of campaigning for a provincial governorship, Didius complained at the province he was offered, although whether this refers to Sicily or Britain is unknown. The orator Domitius Afer sarcastically advised him to think of his country. The next governor of Britain, Quintus Veranius, says on his tombstone that he took the job "although he did not seek it", which has been interpreted as a barbed comment on Didius.
The earliest Welsh name for the city, Caerdyf combines the elements Caer (fort) which refers to the Roman fort established around 75AD, and a second element which is less certain. The antiquarian William Camden recorded the origin of the name as "Caer-Didi" (Didius' Fort). This derives from local beliefs that Didius had built the fort before the arrival of Frontinus in Britain and his construction of numerous supplementary fortifications in South Wales.
Though most modern linguists dismiss this derivation, the Didius connection has remained popular throughout the centuries, appearing in Camden's Britannia (1586), The Beauties of England and Wales (1815), and the writings of Iolo Morgannwg and Taliesin Williams.