Leo, a member of the Bessi tribe from Thrace, was born in AD 401. He had with his wife Verina three children. The oldest was Ariadne who was born before he become emperor, Leontia was born in AD 457, the year he became emperor and an unnamed son who died only 5 months old in AD 463. Ariadne married Leo's successor Zeno, while Leontia married the son of Anthemius the son in law of the emperor of the time Marcian. Leo's early life was all about military, and he had reached the rank of tribune in the regiment of the Mattiarii by 457. When emperor Marcian died in 457, his son-in-law Anthemius appeared the most likely candidate for the throne, but Marcian 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. The senate accepted Aspar's choice and Leo was crowned emperor at AD 457 by the patriarch of Constantinople, Anatolius. This was the first time that the patriarch of Constantinople as well as the army and Senate together were involved in electing an emperor.
Because the previous peace treaty of Theodosius II with the Sassanid Persians still continued, had Leo I three major areas of concern in foreign policy. The Balkans, the Vandals and the Western Empire. With the disintegration of Attila's Empire, two Gothic groups, under Theoderic Strabo and Valamir, moved into the Balkans. Hostilities by Valamir in the western Balkans in 459 led to a negotiated peace with the Romans giving a small subsidy in exchange for peace and a hostage. Also at the same time, Avars, Slavs and Bulgars, began filtering into the Transdanubian region. This in turn put increased pressure on Roman forces holding the Danube.
Leo was to be a harsh persecutor of Christian heretics and the remaining pagans throughout his reign, so it came to no surprise that soon after Leo's acclamation, the early domestic events concerned religion. In 460, the Alexandrians elected a monophysite patriarch, Timothy Aelurus, Leo consulted bishops throughout the east empire about this issue and he removed Timothy Aelurus by force, replacing him with Timothy Salofaciolus. 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 in AD 461. Those Isaurians soon created some form a counterbalance to the overly influential German soldier's of Aspar. Constantinople was devastated by a major fire in 464. During the early part of Leo's reign, the magister militum Aspar was a prominent figure and probably instrumental in Leo's accession. This position was weakened when Aspar's son, Ardabur, was found guilty of treachery in 466 and dismissed from office. Leo then married his eldest daughter, Ariadne, to Zeno, probably in 466, Aspar, however, continued to be influential. In 467, the pagan Isocasius, a court official, was arrested and accused of being a Hellene (i.e. pagan). Although he was acquitted of the charges, this probably led to Leo's issuing a law in 468 which banned pagans from being lawyers.
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. 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. His grandchild at the age of six Leo II, died in the year of his accession. Leo I's son-in-law, Zeno, then became emperor.