Leo was crowned emperor with his brothers Constantine and Alexander as co-emperors to their legal father, Basil I. Leo's biological parentage is debated, as his mother, Eudokia Ingerina, was the mistress of the Emperor Michael III and was forcibly married to Basil, Michael's favourite, to keep her in the palace. She was reputedly already pregnant with Leo when she was married to Basil. Regardless of the reality of the situation, upon becoming senior emperor alongside his surviving brother, Alexander, in 886, Leo had Michael's body exhumed and reburied in the mausoleum of Constantine at the Church of the Holy Apostles, and, while Basil was alive, relations between Basil and Leo were often sour. This has led many modern scholars to suggest that Leo believed himself to be the son of Michael III. Leo's son, Constantine VII, however, actively promoted Basil as his grandfather, a righteous emperor who freed the Empire from a tyrant (Michael III) and founded Constantine VII's imperial line.

Leo's particular preoccupation as emperor was the birth of a son. He had been married to Theophano, whom he disliked, by Basil. After Basil's death, Leo married Zoe Zaoutzina, who died after giving Leo a daughter, Anna. Leo then married a third time, raising church eyebrows, this time to a noblewoman called Eudokia Baiana, who died leaving no surviving children. Finally Leo took up with a mistress, Zoe Karbonopsina ("the black-eyed"), who gave him an illegitimate son, Constantine, causing Leo to marry her and legitimise their son. Since a fourth marriage was uncannonical, however, Leo came into conflict with the Patriarch of Constantinople, and so it is speculated that the unnamed emperor doing penance before Christ in a tenth-century mosaic in the Hagia Sophia is meant to be Leo.

It was also during Leo's reign that Sicily was finally lost to the Empire.

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