In both medieval East and West, Constantine was presented as an ideal ruler, the standard against which any king or Emperor could be measured. The Renaissance rediscovery of anti-Constantinian sources prompted a re-evaluation of Constantine's career. The German humanist Johann Löwenklau, discoverer of Zosimus' writings, published a Latin translation thereof in 1576. In its preface, he argued that Zosimus' picture of Constantine was superior to that offered by Eusebius and the Church historians, offered a more balanced view. Cardinal Caesar Baronius, a man of the Counter-Reformation, criticized Zosimus, favoring Eusebius' account of the Constantinian era. Baronius' Life of Constantine (1588) presents Constantine as the model of a Christian Prince. For his History of the Decline and Fall of the Roman Empire (1776–89), Edward Gibbon, aiming to unite the two extremes of Constantinian scholarship, offered a portrait of Constantine built on the contrasted narratives of Eusebius and Zosimus. In a form that parallels his account of the empire's decline, Gibbon presents a noble war hero who transforms into an Oriental despot in his old age: "a hero ... degenerating into a cruel and dissolute monarch".

Modern interpretations of Constantine's rule begin with Jacob Burckhardt's The Age of Constantine the Great (1853, rev. 1880). Burckhardt's Constantine is a scheming secularist, a Politician who manipulates all parties in a quest to secure his own power. Henri Grégoire, writing in the 1930s, followed Burckhardt's evaluation of Constantine. For Grégoire, Constantine developed an interest in Christianity only after witnessing its political usefulness. Grégoire was skeptical of the authenticity of Eusebius' Vita, and postulated a pseudo-Eusebius to assume responsibility for the vision and conversion narratives of that work. Otto Seeck, in Geschichte des Untergangs der antiken Welt (1920–23), and André Piganiol, in L'empereur Constantin (1932), wrote against this historiographic tradition. Seeck presented Constantine as a sincere war hero, whose ambiguities were the product of his own naïve inconsistency. Piganiol's Constantine is a philosophical monotheist, a child of his era's religious syncretism. Related histories by A.H.M. Jones (Constantine and the Conversion of Europe, 1949) and Ramsay MacMullen (Constantine, 1969) gave portraits of a less visionary, and more impulsive, Constantine.

The death of Maximian required a shift in Constantine's public image. He could no longer rely on his connection to the elder Emperor Maximian, and needed a new source of legitimacy. In a speech delivered in Gaul on 25 July 310 AD, the anonymous orator reveals a previously unknown dynastic connection to Claudius II, a 3rd Century Emperor famed for defeating the Goths and restoring order to the empire. Breaking away from tetrarchic Models, the speech emphasizes Constantine's ancestral prerogative to rule, rather than principles of imperial equality. The new ideology expressed in the speech made Galerius and Maximian irrelevant to Constantine's right to rule. Indeed, the orator emphasizes ancestry to the exclusion of all other factors: "No chance agreement of men, nor some unexpected consequence of favor, made you Emperor," the orator declares to Constantine.

These later accounts were more willing to present Constantine as a genuine convert to Christianity. Beginning with Norman H. Baynes' Constantine the Great and the Christian Church (1929) and reinforced by Andreas Alföldi's The Conversion of Constantine and Pagan Rome (1948), a historiographic tradition developed which presented Constantine as a committed Christian. T. D. Barnes's seminal Constantine and Eusebius (1981) represents the culmination of this trend. Barnes' Constantine experienced a radical conversion, which drove him on a personal crusade to convert his empire. Charles Matson Odahl's recent Constantine and the Christian Empire (2004) takes much the same tack. In spite of Barnes' work, arguments over the strength and depth of Constantine's religious conversion continue. Certain themes in this school reached new extremes in T.G. Elliott's The Christianity of Constantine the Great (1996), which presented Constantine as a committed Christian from early childhood. A similar view of Constantine is held in Paul Veyne's recent (2007) work, Quand notre monde est devenu chrétien, which does not speculate on the origins of Constantine's Christian motivation, but presents him, in his role as Emperor, as a religious revolutionary who fervently believed himself meant "to play a providential role in the millenary economy of the salvation of humanity".

During the medieval period, Britons regarded Constantine as a king of their own people, particularly associating him with Caernarfon in Gwynedd. While some of this is owed to his fame and his proclamation as Emperor in Britain, there was also confusion of his family with Magnus Maximus's supposed wife Saint Elen and her son, another Constantine (Welsh: Custennin). In the 12th century Henry of Huntingdon included a passage in his Historia Anglorum that the Emperor Constantine's mother was a Briton, making her the daughter of King Cole of Colchester. Geoffrey of Monmouth expanded this story in his highly fictionalized Historia Regum Britanniae, an account of the supposed Kings of Britain from their Trojan origins to the Anglo-Saxon invasion. According to Geoffrey, Cole was King of the Britons when Constantius, here a senator, came to Britain. Afraid of the Romans, Cole submitted to Roman law so long as he retained his kingship. However, he died only a month later, and Constantius took the throne himself, marrying Cole's daughter Helena. They had their son Constantine, who succeeded his father as King of Britain before becoming Roman Emperor.

Latin Rite Catholics considered it inappropriate that Constantine was baptized only on his death-bed and by an unorthodox bishop, as it undermined the authority of the Papacy. Hence, by the early fourth century, a legend had emerged that Pope Sylvester I (314–335) had cured the pagan Emperor from leprosy. According to this legend, Constantine was soon baptized, and began the construction of a church in the Lateran Palace. In the eighth century, most likely during the pontificate of Stephen II (752–757), a document called the Donation of Constantine first appeared, in which the freshly converted Constantine hands the temporal rule over "the city of Rome and all the provinces, districts, and cities of Italy and the Western regions" to Sylvester and his successors. In the High Middle Ages, this document was used and accepted as the basis for the Pope's temporal power, though it was denounced as a forgery by Emperor Otto III and lamented as the root of papal worldliness by the poet Dante Alighieri. The 15th century philologist Lorenzo Valla proved the document was indeed a forgery.

During his life and those of his sons, Constantine was presented as a paragon of virtue. Pagans such as Praxagoras of Athens and Libanius showered him with praise. When the last of his sons died in 361, however, his nephew (and son-in-law) Julian the Apostate wrote the satire Symposium, or the Saturnalia, which denigrated Constantine, calling him inferior to the great pagan emperors, and given over to luxury and greed. Following Julian, Eunapius began—and Zosimus continued—a historiographic tradition that blamed Constantine for weakening the Empire through his indulgence to the Christians.