Like Augustine himself, Severus a native of Thagaste, and a fellow student with Augustine. As a friend, he was in Milan when Augustine was there, and was at that time a baptised Christian. He agreed to return to Thagaste with Augustine, who wanted to form a Christian lay community there. Other persons willing to move from Milan to Thagaste were Alypius, who was another friend of Augustine and the one who was baptised beside Augustine, Adeodatus, who was the son of Augustine, and Monica, who was the mother of Augustine. A final member of the intended travelling party was Evodius. Severus was a member of the lay community formed by Augustine at Thagaste. After Augustine moved to Hippo to become a priest, Severus also became a priest and joined the priestly community that Augustine attached to the house of the bishop at Hippo. In Sermon 356 there is a deacon by the name of Severus mentioned by Augustine as being there.
When later a bishop himself, Severus followed the example of Augustine and lived in community in his episcopal house at Milevi (or Milevus). Early in his episcopate, probably in the year 401, Augustine, Alypius, and Samsucius had to explain their conduct in the matter of Timotheus and to call on Severus to accept their explanation (Letters 62 and 63). This temporary mistake in understanding did not interrupt his friendship with Augustine, nor cause any ill-will on his part towards Timotheus. (Ennarationes in Psalmos 95. 1; City of God 21, 4) In a letter somewhat later, perhaps in the year 406, addressed to Novatus, Augustine regretted often being unable to see his old friend. Severus wrote infrequently, and then chiefly on business. This was not because of a lack of goodwill but from the pressure of his work. (Letter 84) He also exchanged letters and friendly messages with Paulinus of Nola (Letters 31, 9 and 32, 1).
In about the year 409 he wrote to Augustine expressing his great delight in his writings. Severus wrote that the thoughts of Augustine led him to greater love of God. He asked Augustine to write in return (Letter 109). Augustine replied, insisting that in their correspondence he himself was the debtor. At that time, in the autumn of the year 416, a council of fifty nine bishops from throughout Numidia met at Mileve, which was the episcopal seat of Severus. Augustine attended this council. While there Augustine and his associates Alypius, Possidius, Evodius and also possibly their host bishop,Severus, reinforced their counciliar decree against Pelagius. They did this by taking the step of writing directly to the Pope. (Augustine, Letters 175 and 176). Their request for papal backing of their condemnation of Pelagius was supported in a reply dated 27th January 417 from Pope Innocent I, who expelled Pelagius from the church.
Little more is known about Severus. He died about the year 426, four years before Augustine died. He was mentioned in the City of God in an illustration that Augustine was making: "Yet far more astonishing is what I heard about this stone from my brother in the episcopate, Severus bishop of Milevis. He told me that Bathanarius, once count of Africa, when the bishop was dining with him, produced a magnet, and held it under a silver plate on which he placed a bit of iron; then as he moved his hand with the magnet underneath the plate, the iron upon the plate moved about accordingly. The intervening silver was not affected at all, but precisely as the magnet was moved backwards and forwards below it, no matter how quickly, so was the iron attracted above…." (City of God 21, 4)