The Vita Basilii records that "Basilius imperator" was "ex regione Macedonum…originem…ad Armenios, Arsacidæ genere", recording in a later passage that he and his parents were captured by the Bulgars while Krum was "Bulgarorum princeps" (he died in 814). Settipani suggests that Basileios was born in . He cites no source on which this suggestion is based, but the hypothesis is inconsistent with the family’s capture by Krum Tsar of the Bulgarians. Symeon Magister records that "Basilius genere Macedo" was 25 years old when he is first introduced into the narrative. It is not possible to date this text with any accuracy, but, on the assumption that Basileios arrived at the royal palace in the early 850s, he must have been born much later than before the death of Krum Tsar of Bulgaria if the passage is correct. Basileios went to Constantinople where he was a groom in the imperial stables and became the favourite of Emperor Mikhael III. Basileios murdered Bardas (the emperor's maternal uncle and chief adviser) 21 Apr 865 during a campaign against the Arabs in Crete. Mikhael III created Basileios parakoimomenos (equivalent to Lord Chamberlain, literally "one who sleeps near [the emperor]") in 865, and magistros in 866 when he also adopted him.
Theophanes Continuatus records that Emperor Mikhael invested "Basilium" with "magistri dignitate". He crowned him co-emperor 26 May 866, but Basileios ordered the murder of Emperor Mikhael in the following year and succeeded 24 Sep 867 as Emperor BASILEIOS I. He recalled Ignatius, installed him as Patriarch of Constantinople 23 Nov 867, and re-established relations with the Church of Rome. His fleet relieved the siege of Dubrovnik by the Arabs in 868, after which Emperor Basileios formed an alliance with Emperor Ludwig II, sealed by the betrothal of his son to Emperor Ludwig's daughter, with a view to prevent further Arab advances in Italy and Dalmatia. After this alliance broke down, Basileios turned his attention eastwards, capturing Zapetra and Samosate in the Euphrates region in 873, but suffering a defeat at Melitene. In administrative affairs, he conceived a project to rewrite the legal codes of Justinian which, although never finalised, formed the basis for his son's subsequent legislative work.
He was mortally wounded in a hunting accident. Emperor Konstantinos VII's De Ceremoniis Aulæ records that "Basilium et eius uxorem Eudociam et filiam Alexandrum" were buried in the church of the Holy Apostles.
Married firstly (repudiated 865) Maria, daughter of ---. Symeon Magister records that "Basilium" repudiated "Mariam coniugem" in the eleventh year of the reign of Emperor Mikhael III, and that she was sent back with gold to her country of origin. Georgius Monachus Continuatus records that "Basilium…uxore Maria" was sent back "in Macedoniam ad suos" after her divorce. 1