Augustine won the job and headed north to take up his position in late 384. At the age of thirty, he had won the most visible academic position in the Latin world – at a time when such posts gave ready access to political careers. During this period, although Augustine showed some fervor for Manichaeism, he was never an initiate or "elect" but remained an "auditor", the lowest level in the sect's hierarchy.

While he was in Milan, Augustine's life changed. While still at Carthage, he had begun to move away from Manichaeism, in part because of a disappointing meeting with the Manichean Bishop, Faustus of Mileve, a key exponent of Manichaean theology. In Rome, he is reported to have completely turned away from Manichaeanism, and instead embraced the skepticism of the New Academy movement. At Milan, his mother pressured him to become a Christian. Augustine's own studies in Neoplatonism were also leading him in this direction, and his friend Simplicianus urged him that way as well. But it was the bishop of Milan, Ambrose, who had most influence over Augustine. Ambrose was a master of rhetoric like Augustine himself, but older and more experienced.

Augustine's mother had followed him to Milan and he allowed her to arrange a society marriage, for which he abandoned his concubine. It is believed that Augustine truly loved the woman he had lived with for so long. In his "Confessions," he expressed how deeply he was hurt by ending this relationship, and also admitted that the experience eventually produced a decreased sensitivity to pain over time. However, he had to wait two years until his fiancee came of age, so despite the grief he felt over leaving "The One", as he called her, he soon took another concubine. Augustine eventually broke off his engagement to his eleven year old fiancee, but never renewed his relationship with "The One" and soon left his second concubine. It was during this period that he uttered his famous prayer, "Grant me chastity and continence, but not yet" (da mihi castitatem et continentiam, sed noli modo).

In the summer of 386, after having read an account of the life of Saint Anthony of the Desert, which greatly inspired him, Augustine underwent a profound personal crisis, leading him to convert to Christianity, abandon his career in rhetoric, quit his teaching position in Milan, give up any ideas of marriage, and devote himself entirely to serving God and to the practices of priesthood, which included celibacy. According to Augustine his conversion was prompted by a childlike voice he heard telling him in a sing song voice, "tolle, lege" ("take up and read"):

I cast myself down I know not how, under a certain fig tree, giving full vent to my tears; and the floods of mine eyes gushed out an acceptable sacrifice to Thee. And, not indeed in these words, yet to this purpose, spake I much unto Thee: and Thou, O Lord, how long? how long, Lord, wilt Thou be angry for ever? Remember not our former iniquities, for I felt that I was held by them. I sent up these sorrowful words: How long, how long, "to-morrow, and tomorrow?" Why not now? why not is there this hour an end to my uncleanness? So was I speaking and weeping in the most bitter contrition of my heart, when, lo! I heard from a neighbouring house a voice, as of boy or girl, I know not, chanting, and oft repeating, "Take up and read; Take up and read. " Instantly, my countenance altered, I began to think most intently whether children were wont in any kind of play to sing such words: nor could I remember ever to have heard the like. So checking the torrent of my tears, I arose; interpreting it to be no other than a command from God to open the book, and read the first chapter I should find. For I had heard of Antony, that coming in during the reading of the Gospel, he received the admonition, as if what was being read was spoken to him: Go, sell all that thou hast, and give to the poor, and thou shalt have treasure in heaven, and come and follow me: and by such oracle he was forthwith converted unto Thee. Eagerly then I returned to the place where Alypius was sitting; for there had I laid the volume of the Apostle when I arose thence. I seized, opened, and in silence read that section on which my eyes first fell: Not in rioting and drunkenness, not in chambering and wantonness, not in strife and envying; but put ye on the Lord Jesus Christ, and make not provision for the flesh, in concupiscence. No further would I read; nor needed I: for instantly at the end of this sentence, by a light as it were of serenity infused into my heart, all the darkness of doubt vanished away.

— The Confessions of Saint Augustine, Book VIII, Paragraphs 28 and 29.

The volume Augustine read was Paul's Epistle to the Romans (Romans 13: 13-14). He wrote an account of his conversion in his Confessions, which became a classic of Christian theology. Ambrose baptized Augustine, along with his son, Adeodatus, on Easter Vigil in 387 in Milan, and a year later they returned to Africa. Also in 388 he completed his apology On the Holiness of the Catholic Church. On the way back to Africa Augustine's mother died, and Adeodatus soon after.

Upon his return to north Africa Augustine sold his patrimony and gave the money to the poor. The only thing he kept was the family house, which he converted into a monastic foundation for himself and a group of friends. In 391 he was ordained a priest in Hippo Regius (now Annaba, in Algeria). He became a famous preacher (more than 350 preserved sermons are believed to be authentic), and was noted for combating the Manichaean religion, to which he had formerly adhered.

In 395 he was made coadjutor Bishop of Hippo, and became full Bishop shortly thereafter. He remained in this position until his death in 430. Augustine worked tirelessly in trying to convince the people of Hippo to convert to Christianity. He left his monastery, but continued to lead a monastic life in the episcopal residence. He left a regula his monastery that has led him to be designated the "patron saint of regular clergy".

Much of Augustine's later life was recorded by his friend Possidius, bishop of Calama (present day Guelma, Algeria), in his Sancti Augustini Vita. Possidius admired Augustine as a man of powerful intellect and a stirring orator who took every opportunity to defend Christianity against its detractors. Possidius also described Augustine's personal traits in detail, drawing a portrait of a man who ate sparingly, worked tirelessly, despised gossip, shunned the temptations of the flesh, and exercised prudence in the financial stewardship of his see.

Shortly before Augustine's death, Roman Africa was invaded by the Vandals, a Germanic tribe that that had converted to Arianism. The Vandals besieged Hippo in the spring of 430, when Augustine entered his final illness. According to Possidius one of the few miracles attributed to Augustine took place during the siege. While Augustine was confined to his sick bed, a man petitioned him that he might lay his hands upon a relative who was ill. Augustine replied that if he had any power to cure the sick, he would surely have applied it on himself first. The visitor declared that he was told in a dream to go to Augustine so that his relative would be made whole. When Augustine heard this, he no longer hesitated, but laid his hands upon the sick man, who departed from Augustine's presence healed.

Possidius also gives a first hand account of Augustine's death, which occurred on August 28, 430, while Hippo was still besieged. Augustine spent his final days in prayer and repentance, requesting that the penitential Psalms of David be hung on his walls so that he could read them. He directed that the library of the church in Hippo and all the books therein should be carefully preserved. Shortly after his death the Vandals lifted the siege of Hippo, but they returned not long thereafter and burned the city. They destroyed all of it but Augustine's cathedral and library, which they left untouched.

According to Bede's True Martyrology, Augustine's body was later removed to Cagliari, Sardinia, by the Catholic bishops expelled from North Africa by Huneric. Around 720 his remains were moved again by Peter, bishop of Pavia and uncle of the Lombard king Liutprand, to the church of San Pietro in Ciel d'Oro, in order to save them from frequent coastal raids by Moors and Berbers. In January 1327 Pope John XXII issued the papal bull Veneranda Santorum Patrum, in which he appointed the Augustinians guardians of the tomb of Augustine, which was remade in 1362 and elaborately carved with bas - treliefs of scenes from Augustine's life. By that time, however, the actual remains of Augustine could not be authenticated. Stonemasons working in the crypt altar removed paving blocks and discovered a marble box. Within it were other boxes; in the third box were fragments of wood, numerous bones and bone fragments, and glass vials. Some of the workers later claimed to have seen the name "Augustine" written in charcoal on the top of the box. A factor complicating the authentication of the remains was that San Pietro was shared by two Augustinian religious orders in bitter rivalry. The Augustinians were expelled from Pavia in 1700, taking refuge in Milan with the relics of Augustine, and the disassembled Arca, which were removed to the cathedral there. San Pietro fell into disrepair and was a military magazine during the Napoleonic occupation of the city. It was finally rebuilt in the 1870s, under the urging of Agostino Gaetano Riboldi, and reconsecrated in 1896 when the relics of Augustine and the shrine were once again reinstalled.

Augustine was one of the most prolific Latin authors in terms of surviving works, and the list of his works consists of more than a hundred separate titles. They include apologetic works against the heresies of the Arians, Donatists, Manichaeans and Pelagians, texts on Christian doctrine, notably De Doctrina Christiana (On Christian Doctrine), exegetical works such as commentaries on Book of Genesis, the Psalms and Paul's Letter to the Romans, many sermons and letters, and the Retractationes (Retractions), a review of his earlier works which he wrote near the end of his life. Apart from those, Augustine is probably best known for his Confessiones (Confessions), which is a personal account of his earlier life, and for De civitate dei (Of the City of God, consisting of 22 books), which he wrote to restore the confidence of his fellow Christians, which was badly shaken by the sack of Rome by the Visigoths in 410. His De trinitate (On the Trinity), in which he developed what has become known as the 'psychological analogy' of the Trinity, is also among his masterpieces, and arguably one of the greatest theological works of all time. He also wrote On Free Choice Of The Will (De libero arbitrio), addressing why God gives humans free will that can be used for evil.

Augustine was a bishop, priest, and father who remains a central figure, both within Christianity and in the history of Western thought, and is considered by modern historian Thomas Cahill to be the first medieval man and the last classical man. In both his philosophical and theological reasoning, he was greatly influenced by Stoicism, Platonism and Neo - platonism, particularly by the work of Plotinus, author of the Enneads, probably through the mediation of Porphyry and Victorinus (as Pierre Hadot has argued). Although he later abandoned Neoplatonism some ideas are still visible in his early writings. His generally favourable view of Neoplatonic thought contributed to the "baptism" of Greek thought and its entrance into the Christian and subsequently the European intellectual tradition. His early and influential writing on the human will, a central topic in ethics, would become a focus for later philosophers such as Schopenhauer and Nietzsche. In addition, Augustine was influenced by the works of Virgil (known for his teaching on language), Cicero (known for his teaching on argument), and Aristotle (particularly his Rhetoric and Poetics).

Augustine's concept of original sin was expounded in his works against the Pelagians. However, St. Thomas Aquinas took much of Augustine's theology while creating his own unique synthesis of Greek and Christian thought after the widespread rediscovery of the work of Aristotle. Augustine's doctrine of efficacious grace found eloquent expression in the works of Bernard of Clairvaux; also Reformation theologians such as Martin Luther and John Calvin would look back to him as their inspiration.

Augustine was canonized by popular acclaim, and later recognized as a Doctor of the Church in 1298 by Pope Boniface VIII. His feast day is August 28, the day on which he died. He is considered the patron saint of brewers, printers, theologians, sore eyes, and a number of cities and dioceses.

The latter part of Augustine's Confessions consists of an extended meditation on the nature of time. Even the agnostic philosopher Bertrand Russell was impressed by this. He wrote, "a very admirable relativistic theory of time. ... It contains a better and clearer statement than Kant's of the subjective theory of time - a theory which, since Kant, has been widely accepted among philosophers." Catholic theologians generally subscribe to Augustine's belief that God exists outside of time in the "eternal present"; that time only exists within the created universe because only in space is time discernible through motion and change. His meditations on the nature of time are closely linked to his consideration of the human ability of memory. Frances Yates in her 1966 study The Art of Memory argues that a brief passage of the Confessions, 10.8.12, in which Augustine writes of walking up a flight of stairs and entering the vast fields of memory clearly indicates that the ancient Romans were aware of how to use explicit spatial and architectural metaphors as a mnemonic technique for organizing large amounts of information.

Augustine's philosophical method, especially demonstrated in his Confessions, has had continuing influence on Continental philosophy throughout the 20th century. His descriptive approach to intentionality, memory, and language as these phenomena are experienced within consciousness and time anticipated and inspired the insights of modern phenomenology and hermeneutics. Edmund Husserl writes: "The analysis of time - consciousness is an age old crux of descriptive psychology and theory of knowledge. The first thinker to be deeply sensitive to the immense difficulties to be found here was Augustine, who laboured almost to despair over this problem." Martin Heidegger refers to Augustine's descriptive philosophy at several junctures in his influential work, Being and Time. Hannah Arendt began her philosophical writing with a dissertation on Augustine's concept of love, Der Liebesbegriff bei Augustin (1929): "The young Arendt attempted to show that the philosophical basis for vita socialis in Augustine can be understood as residing in neighbourly love, grounded in his understanding of the common origin of humanity." Jean Bethke Elshtain in Augustine and the Limits of Politics finds likeness between Augustine and Arendt in their concepts of evil: "Augustine did not see evil as glamorously demonic but rather as absence of good, something which paradoxically is really nothing. Arendt ... envisioned even the extreme evil which produced the Holocaust as merely banal [in Eichmann in Jerusalem]." Augustine's philosophical legacy continues to influence contemporary critical theory through the contributions and inheritors of these 20th century figures.

According to Leo Ruickbie, Augustine's arguments against magic, differentiating it from miracle, were crucial in the early Church's fight against paganism and became a central thesis in the later denunciation of witches and witchcraft. According to Professor Deepak Lal, Augustine's vision of the heavenly city has influenced the secular projects and traditions of the Enlightenment, Marxism, Freudianism and Eco - fundamentalism.

On the topic of original sin, Aquinas proposed a more optimistic view of man than that of Augustine in that his conception leaves to the reason, will, and passions of fallen man their natural powers even after the Fall.

While in his pre - Pelagian writings Augustine taught that Adam's guilt as transmitted to his descendants much enfeebles, though does not destroy, the freedom of their will, Protestant reformers Martin Luther and John Calvin affirmed that Original Sin completely destroyed liberty (total depravity).