In 418 he succeeded King Wallia. The Romans had ordered King Wallia to move his people from Iberia to Gaul. As king, Theodoric completed the settlements of the Visigoths in Gallia Aquitania II, Novempopulana and Gallia Narbonensis, and then used the declining power of the Roman Empire to extend his territory to the south.
After the death of Emperor Honorius and the usurpation of Joannes in 423 internal power struggles broke out in the Roman Empire. Theodoric used this situation and tried to capture the important road junction Arelate, but the Magister militum Aëtius, who was assisted by the Huns, was able to save the city.
The Visigoths concluded a treaty and were given Gallic noblemen as hostages. The later Emperor Avitus visited Theodoric, lived at his court and taught his sons.
Because the Romans had to fight against the Franks, who plundered Cologne and Trier in 435, and because of other events Theodoric saw the chance to conquer Narbo Martius (in 436) to obtain access to the Mediterranean Sea and the roads to the Pyrenees. But Litorius, with the aid of the Huns, could prevent the capture of the city and drove the Visigoths back to their capital Tolosa. The peace offer of Theodoric was refused, but the king won the decisive battle at Tolosa, and Litorius soon died in Gothic imprisonment from the injuries which he had received in this battle. Avitus went – according to the orders of Aëtius – to Tolosa and offered a peace treaty which Theodoric accepted. Perhaps the Romans recognized at that time the sovereignty of the Visigoth state.
A daughter of Theodoric had been married to Huneric, a son of the Vandal ruler Geiseric (in 429?), but Huneric later had ambitions to wed Eudocia, a daughter of the Emperor Valentinian III. He therefore accused the daughter of Theodoric of planning to kill him, and in 444 had her mutilated - her ears and nose cut off - and sent back to her father. This action caused an enmity between the Visigoths and the Vandals.