Justin was born in the Roman province of Illyricum of humble stock sometime around 435. As a teenager, he and two companions fled from an invasion of the Huns in 447, taking refuge in Constantinople. Justin soon joined the army, and because of his ability, rose through the ranks to become a general and commander of the palace guard under the emperor Anastasius I more than a half-century later. Thanks to his position (he commanded the only troops in the city) and gifts of money, Justin was able to secure election as emperor in 518.
A career soldier with little knowledge of statecraft, Justin wisely surrounded himself with trusted advisors. The most prominent of these men, of course, was his nephew Flavius Petrus Sabbatius, whom he adopted as his son and invested with the name Justinian. It is common now to say that Justinian ruled the empire in his uncle's name during this time, thanks to the accounts of the historian Procopius, but there is much evidence to the contrary. In fact, Justinian wasn't named as successor until less than a year before Justin's death.
In 525, Justin repealed a law that effectively prohibited a member of the senatorial class from marrying a woman from a lower class of society, including the theatre, which was considered scandalous at the time. This edict paved the way for Justinian to marry Theodora, a former mime actress, and eventually resulted in a major blurring of the old class distinctions at the Imperial court.
The latter years of his reign were marked by strife between the empire and the Ostrogoths and Persians. In 526, Justin's health began to decline and he formally named Justinian as co-emperor and his successor on April 1, 527. On August 1 of that year, Justin died and was succeeded by Justinian.