The Excubitors were founded by Emperor Leo I (reigned 457–474) c. 460 and numbered 300 men, often recruited from among the sturdy and warlike Isaurians, as part of Leo's effort to counterbalance the influence of the magister militum Aspar and the large Germanic element in the East Roman army. Unlike the older palace regiments of the Scholae Palatinae, which were under the control of the magister officiorum and eventually degenerated to parade-ground formations, the Excubitors long remained a crack fighting force. In addition, while the Scholae were garrisoned throughout Thrace and Bithynia, the Excubitors were billeted in the imperial palace itself and formed practically the only garrison of Constantinople in the 6th century. Their high status is further illustrated by the fact that both officers and ordinary Excubitors were often sent for special missions by the emperors, including diplomatic assignments.
The unit was headed by the Count of the Excubitors (Latin: comes excubitorum; Greek: κόμης τῶν ἐξκουβίτων/ἐξκουβιτόρων, komēs tōn exkoubitōn/exkoubitorōn), who, by virtue of his proximity to the emperor, became an official of great importance in the 6th and 7th centuries. This post, which can be traced up to c. 680, was usually held by close members of the imperial family, often virtual heirs apparent. Thus it was the support of his men that secured Justin I (r. 518–527), who held the post at the time of the death of Anastasius I, his elevation to the throne. Similarly, Justin II (r. 565–578) relied on the support of the Excubitors for his unchallenged accession; their count, Tiberius, was a close friend who had been appointed to the post through Justin's intervention. Tiberius was to be the Emperor's right-hand man throughout his reign, eventually succeeding him as Tiberius II (r. 578–582). He too would be succeeded by his own comes excubitorum, Maurice (r. 582–602). Under Maurice, the post was held by his brother-in-law Philippicus, and under Phocas (r. 602–610) by Priscus. Another powerful occupant was Valentinus, who secured it during the power struggles that accompanied the regency of Empress-dowager Martina in 641, before deposing her and her son Heraklonas and installing Constans II (r. 641–668) as emperor. Valentinus dominated the new regime, but his attempt to become emperor in 644 ended in his being lynched by the mob. The power that went with the position, and the intrigues of men like Priscus and the would-be usurper Valentinus, doomed the post to emasculation and eventual eclipse during the latter half of the 7th century.
After a lapse towards the end of the 7th century and the first half of the 8th century, the Excubitors reappear in historical sources, under a new commander, the Domestic of the Excubitors (δομέστικος τῶν ἐξκουβίτων/ἐξκουβιτόρων, domestikos tōn exkoubitōn/exkoubitorōn) and in a new capacity, as one of the imperial tagmata, the elite professional central army established by Constantine V (r. 741–775). As one of the tagmata, the Excubitors were no longer a palace guard, but a unit actively engaged in military campaigns. At the same time, the tagmata represented a counterbalance to the thematic armies of the provinces and constituted a powerful tool in implementing the iconoclastic policies pursued by Constantine V. Nevertheless, the possibly first commander of the tagma, Strategios Podopagouros, was among the leaders of a failed plot against Constantine V's life in 766, and was executed after its discovery. By the 780s, however, following years of imperial favour and military victories under Constantine V and his son Leo IV the Khazar (r. 775–780), the tagmata had become firm adherents to the iconoclast cause. Within less than two months of Leo V's death in 780, Empress-regent Irene of Athens had to foil an attempt spearheaded by the Domestic of the Excubitors to place Constantine V's exiled second son Nikephoros on the throne, and in 786 Irene forcibly disarmed them and exiled some 1,500 tagmatic soldiers due to their resistance to the restoration of the icons.
The Domestics were originally of strikingly low court rank (mere spatharioi), but they gradually rose to importance: while in the Taktikon Uspensky of c. 842 the Domestic of the Excubitors came behind all the thematic commanders (stratēgoi) in order of precedence, in the Klētorologion of 899, the Domestic is shown as superior to the stratēgoi of the European themes and even to the Eparch of Constantinople. At the same time, the court dignities they held rose to those of prōtospatharios and even patrikios. The Excubitors participated in the disastrous Pliska campaign in 811, when the Byzantine army was routed by Tsar Krum of Bulgaria; the Domestic of the Excubitors fell in the field along with the other senior Byzantine generals, including Emperor Nikephoros I (r. 802–811). The most prominent Domestic of the Excubitors of the period was Michael II the Amorian (r. 820–829), whose supporters overthrew Emperor Leo V the Armenian (r. 813–820) and raised him to the throne. In the latter half of the 10th century, probably under Romanos II (r. 959–963), the regiment, like the senior-most tagma, the Scholae, was split in two units, one for the West and one for the East, each headed by a respective Domestic.
The Excubitors took part in the failed Azaz campaign of 1030, where they were ambushed and dispersed by the Mirdasids, while their commander, the patrikios Leo Choirosphaktes, was taken captive. As with most of the tagmata, the regiment of the Excubitors did not survive the great upheavals of the later 11th century, when foreign invasion and constant civil wars destroyed much of the Byzantine army. The last mention of the Excubitors occurs in Anna Komnene's Alexiad, where they are recorded for the last time as participating at the Battle of Dyrrhachium against the Normans in 1081, under the command of Constantine Opos.