In 1503, Niccolò Machiavelli, though an avowed republican, esteemed Hadrian as an ideal princeps, one of Rome's Five Good Emperors. Friedrich Schiller called Hadrian "the Empire's first servant". Edward Gibbon admired his "vast and active genius" and his "equity and moderation", and considered Hadrian's era as part of the "happiest era of human history". In Ronald Syme's view, Hadrian "was a Führer, a Duce, a Caudillo". According to Syme, Tacitus' description of the rise and accession of Tiberius is a disguised account of Hadrian's authoritarian Principate. According, again, to Syme, Tacitus' Annals would be a work of contemporary history, written "during Hadrian's reign and hating it".

The first modern Historian to produce a chronological account of Hadrian's life, supplementing the written sources with other epigraphical, numismatic, archaeological evidence, was the German 19th-century medievalist Ferdinand Gregorovius. A 1907 biography by Weber, a German nationalist and later Nazi Party supporter, incorporates the same archaeological evidence to produce an account of Hadrian, and especially his Bar Kokhba war, that has been described as ideologically loaded. Epigraphical studies in the post-war period help support alternate views of Hadrian. Anthony Birley's 1997 biography of Hadrian sums up and reflect these developments in Hadrian historiography.

Hadrian's last years were marred by chronic illness. He saw the bar Kokhba revolt as the failure of his panhellenic ideal. His execution of two more senators for their alleged plots against him provoked further resentment. His marriage to Vibia Sabina had been unhappy, and childless; in 138 he adopted Antoninus Pius and nominated him as a successor, on the condition that Antoninus adopt Marcus Aurelius and Lucius Verus as his own heirs. Hadrian died the same year at Baiae, and Antoninus had him deified, despite opposition from the Senate. Edward Gibbon includes him among the Empire's "Five good emperors", a "benevolent dictator"; Hadrian's own senate found him remote and authoritarian. He has been described as enigmatic and contradictory, with a capacity for both great personal generosity and extreme cruelty, and driven by insatiable curiosity, self-conceit, and above all, ambition. Modern interest was revived largely thanks to Marguerite Yourcenar's novel Mémoires d'Hadrien (1951).

In Hadrian's time, there was already a well established convention that one could not write a contemporary Roman imperial history for fear of contradicting what the emperors wanted to say, read or hear about themselves. Fronto's correspondence and works attest to Hadrian's character and the internal politics of his rule. Greek authors such as Philostratus and Pausanias wrote shortly after Hadrian's reign, but confined their scope to the general historical framework that shaped Hadrian's decisions, especially those relating to Greece. Political histories of Hadrian's reign come mostly from later sources, some of them written centuries after the reign itself. The early 3rd-century Roman History by Cassius Dio gave a general account of Hadrian's reign, but the original is lost, and what survives is a brief, Byzantine-era abridgment by the 11th-century monk Xiphilinius, who focussed on Hadrian's religious interests, the Bar Kokhba war, and little else. The principal source for Hadrian's life and reign is one of several late 4th-century imperial biographies, collectively known as the Historia Augusta. The collection as a whole is notorious for its unreliability ("a mish mash of actual fact, cloak and dagger, sword and sandal, with a sprinkling of Ubu Roi"), but most modern historians consider its account of Hadrian to be relatively free of outright fictions, and probably based on sound historical sources, principaly one of a lost series of imperial biographies by the prominent 3rd-century senator Marius Maximus, who covered the reigns of Nerva through to Elagabalus.

Hadrian has been described as the most versatile of all Roman emperors, who "adroitly concealed a mind envious, melancholy, hedonistic, and excessive with respect to his own ostentation; he simulated restraint, affability, clemency, and conversely disguised the ardor for fame with which he burned." His successor Marcus Aurelius, in his Meditations, lists those to whom he owes a debt of gratitude; Hadrian is conspicuously absent. Hadrian's tense, authoritarian relationship with his senate was acknowledged a generation after his death by Fronto, himself a senator, who wrote in one of his letters to Marcus Aurelius that "I praised the deified Hadrian, your grandfather, in the senate on a number of occasions with great enthusiasm, and I did this willingly, too [...] But, if it can be said – respectfully acknowledging your devotion towards your grandfather – I wanted to appease and assuage Hadrian as I would Mars Gradivus or Dis Pater, rather than to love him." Fronto adds, in another letter, that he kept some friendships, during Hadrian's reign, "under the risk of my life" (cum periculo capitis). Hadrian underscored the autocratic character of his reign by counting his dies imperii from the day of his acclamation by the armies, rather than the senate, and legislating by frequent use of imperial decrees to bypass the Senate's approval. The veiled antagonism between Hadrian and the Senate never grew to overt confrontation as had happened during the reigns of overtly "bad" emperors, because Hadrian knew how to remain aloof and avoid an open clash. That Hadrian spent half of his reign away from Rome in constant travel probably helped to mitigate the worst of this permanently strained relationship.

Hadrian's first official post in Rome was as a judge at the Inheritance court, one among many vigintivirate offices at the lowest level of the cursus honorum ("course of honours") that could lead to higher office and a senatorial career. He then served as a military tribune, first with the Legio II Adiutrix in 95, then with the Legio V Macedonica. During Hadrian's second stint as tribune, the frail and aged reigning Emperor Nerva adopted Trajan as his heir; Hadrian was dispatched to give Trajan the news— or most probably was one of many emissaries charged with this same commission. Then he was transferred to Legio XXII Primigenia and a third tribunate. Hadrian's three tribunates gave him some career advantage. Most scions of the older senatorial families might serve one, or at most two military tribunates as a prerequisite to higher office. When Nerva died in 98, Hadrian is said to have hastened to Trajan, to inform him ahead of the official envoy sent by the governor, Hadrian's brother-in-law and rival Lucius Julius Ursus Servianus.