After rendering good service on behalf of the latter in a war with the Arabs in 812, he was summoned in 813 to co-operate in a campaign against the Bulgars. Taking advantage of the disaffection among the troops, he left Michael in the lurch at the Battle of Adrianople and subsequently led a successful revolution against him. Leo justified his usurpation by repeatedly defeating the Bulgars who had been contemplating the siege of Constantinople (814-817).

His vigorous measures of repression against the Paulicians and image-worshippers roused considerable opposition, and after a conspiracy under his friend Michael Psellus had been foiled by the imprisonment of its leader, Leo was assassinated in the palace chapel on Christmas Eve, 820.