He was unanimously elected to the throne to succeed his brother-in-law Alaric, who had been struck down by a fever suddenly in Calabria. King Ataulf's first act was to halt Alaric's southward expansion of the Goths in Italy.
Meanwhile, Gaul had been separated from the western Roman Empire by the usurper Constantine III. So in 411 Constantius, the magister militium (master of military) of the western emperor, Flavius Augustus Honorius, with Gothic auxiliaries under Ulfilas, crushed the Gallic rebellion with a siege of Arles. There Constantine and his son were offered an honorable capitulation— but were beheaded in September on their way to pay homage to Honorius at Ravenna.
In the spring of 412 Constantius pressed Ataulf. Taking the advice of Priscus Attalus—the former emperor whom Alaric had set up at Rome in opposition to Honorius at Ravenna, and who had remained with the Visigoths after he'd been deposed—Ataulf led his followers out of Italy. Moving north into a momentarily pacified Gaul, the Visigoths lived off the countryside in the usual way. Ataulf may have received some additional encouragement in the form of payments in gold from the Emperor Honorius—since Ataulf carried with him as a respected hostage the emperor's half-sister Galla Placidia, who had long been his captive.
Once in Gaul, Ataulf opened negotiations with a new usurper, the Gallic Jovinus. But when the latter ended up instead preferring Sarus, Ataulf's blood enemy among the Gothic nobles, Ataulf broke negotiations off and attacked and killed Sarus. Jovinus then named his brother Sebastianus (Sebastian) as Augustus (co-emperor). This further offended Ataulf, who hadn't been consulted. So he allied his Visigoths with Honorius. Jovinus' troops were defeated in battle, Sebastianus was captured, and Jovinus fled for his life. Ataulf then turned Sebastianus over for execution to Honorius' Gallic praetorian prefect (provincial governor), Postumus Dardanus. After this, Ataulf besieged and captured Jovinus at Valentia (Valence) in 413, sending him to Narbo (Narbonne), where he was executed by Dardanus.
After the heads of Sebastianus and Jovinus arrived at Honorius' court in Ravenna in late August, to be forwarded for display among other usurpers on the walls of Carthage, relations between Ataulf and Honorius improved sufficiently for Ataulf to cement them by marrying Galla Placidia at Narbo in early 414. The nuptials were celebrated with high Roman festivities and magnificent gifts from the Gothic booty. Priscus Attalus gave the wedding speech, a classical epithalamium.
Under Ataulf's rule, the Visigoths couldn't be said to be masters of a settled kingdom until Ataulf took possession of Narbonne and Toulouse in 413. Still, the Visigoths sustained an uneasy client relationship with the western empire. Although Ataulf remained an Arian Christian, his relationship with Roman culture was summed up, from a Catholic Roman perspective, by the words that the contemporary Christian apologist Orosius put into his mouth, Ataulf's Declaration:
Honorius's general Constantius (who would later become Emperor Constantius III), poisoned official relations with Ataulf and gained permission to blockade the Mediterranean ports of Gaul. In reply, Ataulf acclaimed Priscus Attalus as Augustus in Bordeaux in 414. But Constantius' naval blockade was successful and, in 415, Ataulf withdrew with his people into northern Hispania. Attalus fled, fell into the hands of Constantius, and came to a bad end.
Galla Placidia traveled with Ataulf. The infant son, Theodosius, she bore him died in infancy and was buried in Hispania in a silver-plated coffin , thus eliminating an opportunity for a Romano-Visigothic line.
In Hispania, Ataulf imprudently accepted into his service one of the late Sarus' followers, unaware that the man harbored a secret desire to avenge the death of his beloved patron. And so, in the palace at Barcelona, the man brought Ataulf's reign to a sudden end by killing him while he bathed.
Sigeric, the brother of Sarus, immediately became king—for a mere seven days, when he was also murdered and succeeded by Wallia. Under the latter's reign, Galla Placidia was returned to Ravenna where, in 417, at the urging of Honorius, she remarried, her new husband being the implacable enemy of the Goths, Constantius.