Leo was born in AD 401, a member of the tribe of the Bessi in Thrace.
At the death of Marcian his son-in-law Anthemius appeared the most likely candidate for the throne.
However Anthemius did not enjoy the support of Aspar, the powerful eastern 'Master of Soldiers'. Instead Aspar decided on of the commanders in his own army, Leo, who at the time was a tribune with a legion based at Selymbria (Silivri).
The senate dared not reject Aspar's choice and Leo was crowned emperor by the patriarch of Constantinople, Anatolius.
Leo was to be a harsh persecutor of Christian heretics and the remaining pagans throughout his reign.
Aspar remained the real power within the eastern empire for another six or seven years. However, during this time Leo was not entirely his puppet. Most notably was Leo's intent on self-determination more clear as in his recruitment of a large number of Isaurians into a newly created imperial guard (excubitores) in AD 461.
These Isaurians soon created some form a counterbalance to the overly influential German soldier's of Aspar.
After a raid by Geiseric's Vandals on the Peloponnese in Greece in AD 467, Leo came to realize the scale of the threat posed by the barbarians in Carthage and the necessity for joint action by the two empires against them.
Hence Leo intervened in the west after the death of Majorian to see Anthemius (the very son-in-law of Marcian whom he had beaten to the eastern throne) placed on the western throne, instead of Geiseric's choice Olybrius.
Next, in AD 468 Leo sent a large naval exhibition in a combined effort with western forces against the Vandals. The fleet was commanded by Basiliscus, the brother of Leo's wife, Aelia Verina.
But the fleet suffered a crushing defeat at the hands of Geiseric, and another expedition sent in AD 470 met with the same fate.
The effect was disastrous, as it not only confirmed the Vandals in the eyes of the Romans as all-conquering, but it also almost bankrupted the eastern treasury.
Aspar was accused by some of having aided Geiseric by treason, but this is highly unlikely.
Aspar's main opponent at court was Zeno, an Isaurian who in AD 469 was made 'Master of Soldiers'.
It was in AD 470 when Zeno was in Thrace fighting the Huns that Aspar used his opponent's absence to persuade Leo to grant his son Patricius the long promised rank of Caesar as well as marriage to Leo's daughter Leontia.
The result was public outrage as Patricius, like his father, was a Christian of Arian creed and so officially a heretic.
Aware to the danger Aspar, tried to win over Zeno's Isaurian soldiers at Constantinople to help secure his position. Though this news of Aspar's attempt of winning over his power base reached Zeno in Thrace in AD 471 and he at once returned to Chalcedon from where he, close to Constantinople, could influence matters.
Aspar and his son and general Ardaburius needed to flee into the safety of a church, so fierce was the backlash brought about by Zeno's return. Though despite Leo's personal assurances of their safety, they were both killed, most likely at Zeno's behest.
This in turn caused outrage among Aspar's supporters. An enraged officer of Aspar's named Ostrus even broke into the palace with a party of soldiers, seeking the emperor. But the Isaurian guards drove him back out and he fled to Thrace.
Another consequence of Aspar's fall was the rebellion of the Ostrogothic chieftain Theodoric Strabo. Seeing the most powerful figure removed, he recognized Constantinople's moment of weakness. He rampaged through the Balkans, devastating the cities of Philippopolis (Plovdiv) and Arcadiopolis (Lüleburgaz), until Leo relented, recognized him as king and agreed to pay him an annual subsidy, on condition that he and his Goths would hence fight for the empire. Theodoric Strabo was also granted the post of 'Master of Soldiers', although it was no longer to be the position of the power, men like Zeno and Aspar had wielded when holding it. For Leo was determined to see an end to the German domination of his empire, and hence looked to the Isaurians to act as the eastern power base.
In October AD 473 Leo I raised his grandson, a boy born to Zeno and his daughter Ariadne, to the rank of Augustus as Leo II.
Shortly after this, Leo became very ill and died on the 18 January AD 474.