Starting in childhood, his reign over the Roman Empire was one of the longest, but was dominated by civil wars among powerful generals and the barbarian invasions.

The augusta Galla Placidia had great influence during her son's rule, as did the military commander Flavius Aetius, who defended the western empire against Germanic and Hunnic invasions.

Attila the Hun repeatedly menaced Valentinian's domains, being repulsed by a coalition under Aetius's leadership at the Battle of the Catalaunian Plains and calling off a subsequent invasion after negotiations led by Pope Leo I.

Valentinian was born in Ravenna, the capital of the Western Roman Empire, as the only son of Galla Placidia and Constantius III, who briefly ruled as emperor in 421.

His mother had previously been married to Ataulf of the Visigoths, and had borne a son, Theodosius, in Barcelona in 414; but the child had died early in the following year, thus eliminating an opportunity for a Romano-Visigothic line.

[3] After the death of Constantius in 421, court intrigue forced Galla Placidia to flee from Honorius and move to Constantinople, where she, Valentinian and Honoria were taken in by Theodosius.

On 23 October 425, after Joannes had been defeated in a combined naval and land campaign, Helion, the eastern patricius et magister officiorum, installed Valentinian as augustus in Rome.

[7] Given his minority, the new augustus ruled under the influence of his mother Galla Placidia, one of whose first acts was to install Felix as the magister utriusque militiae in the west.

Weakened, Felix was unable to resist Aetius who, with the support of Galla Placidia, replaced him as magister militum praesentalis in 429, before having him killed in 430.

[18] Concerned by this turn of events and determined to hold onto the African provinces at all costs, the court at Ravenna sought reconciliation with Bonifatius, who agreed in 430 to affirm his allegiance to Valentinian III and stop the Vandal king Gaiseric.

[20] As a consequence, in 435, Valentinian was forced to conclude a peace with Gaiseric, whereby the Vandals kept all their possessions in North Africa in return for a payment of tribute to the empire,[21] while the Huns were granted new territory in Pannonia Savia to occupy.

[25] With Aetius occupied in Gaul, Valentinian was unable to do anything to prevent the Vandals completely overrunning the remaining western African provinces, culminating in the fall of Carthage on 19 October 439.

[27] By 440, Vandal fleets were ravaging Sicily and Aetius coordinated a joint response with the eastern court, with large numbers of Roman troops sent to defend the island from Gaiseric.

[32] This loss of territory caused severe financial problems, with the Roman state openly acknowledging that there was insufficient revenue to meet its military needs.

[34] In the 440s Valentinian made the Hunnic chieftain Attila honorary magister militum of the western empire, hoping thereby to reduce the threat the Huns posed to the Danubian provinces.

[35][36] In 449, Honoria wrote to Attila, offering him half the western empire if he would rescue her from an unwanted political marriage arranged by her brother Valentinian.

In 450 he secured peace with the eastern court and entered the Gallic provinces, having allegedly been bribed by the Vandal king Gaiseric to attack Gaul's population of Visigoths.

[43] The ancient historian Priscus reported that Aetius was presenting a financial statement before the Emperor when Valentinian suddenly leapt from his throne and accused him of drunken depravity.

[47] The day after the assassination Petronius Maximus had himself proclaimed emperor by the remnants of the Western Roman army after paying a large donative.

[48] He was not as prepared as he thought to take over and stabilize the depleted empire, however; after a reign of only 11 weeks, Maximus was stoned to death by a Roman mob.

He is described as spoiled, pleasure-loving, and heavily influenced by sorcerers and astrologers, but also devoted to religion, contributing to churches of Saint Lawrence in both Rome and Ravenna.