here on an Eastern orthodox site. And here is an encyclical by Blessed John XXIII on St. Leo.
So, OK, he was also pope and a doctor of the church. But he was a deacon when he was elected to the papacy.
In addition to being a well-respected leader in his own day, and so important in church history that some historians credit him for shaping the way the Christian church was to operate throughout the Middle Ages, he was one of at least 38 deacons of Rome to be elected directly to the papacy. This was, of course, before the "cursus honorum" became the norm. At this time in history, in many (most?) dioceses, when the bishop died, it was usually the late bishop's "right hand man" -- his deacon -- who would be elected to succeed him.