The title imperator dates back to the Roman Republic. One of the most signal marks of distinction which a commander could receive under the republic was the laurel leaf, with which he was crowned when his soldiers, after a victory, saluted him as imperator. It was a generic title for Roman commanders.

The office of Roman Emperor went through a complex evolution over the centuries of its existence. During its earliest phase, the Principate, the reality of autocratic rule was masked behind the forms and conventions of oligarchic self-government inherited from the Roman Republic. The emperor had no specific office unless he chose to occupy the Republican office of consul.

The Crisis of the Third Century marked the end of the Principate, the early phase of Imperial Roman government. A series of soldiers, the Barracks Emperors, assumed the highest office, leading to the breakdown of the previous system of Imperial government, in which the Emperor had functioned within the fiction of a preservation of the old republican forms of government. The crisis came to a close with Diocletian, who reformed the Imperial office and initiated the period known as the Dominate.

The accession to the purple on November 20, 284, of Diocletian, the lower-class, Greek-speaking Dalmatian commander of Carus's and Numerian's household cavalry (protectores domestici), marked a major departure from traditional Roman constitutional theory regarding the Emperor, who was nominally first among equals during the Principate. Diocletian introduced Oriental despotism into Imperial dignity; whereas before Emperors had worn only a purple toga (toga picta) and greeted with deference, Diocletian wore jewelled robes and shoes, and required those who greeted him to kneel and kiss the hem of his robe (adoratio). In many ways, Diocletian was the first monarchical Emperor, and this is symbolised by the fact that the word dominus ("Lord") rapidly replaced princeps as the favoured word for referring to the Emperor.

The office of Roman Emperor underwent significant turbulence in the fourth and fifth centuries, after assuming the trappings of Eastern despotism during the Dominate. In the West, its holders became puppets of a succession of barbarian kings. In the East, it consolidated its new autocratic trappings as it transformed into the office of Byzantine Emperor.