Ambrose was born around 339 in Trier, then the seat of a praetorian prefecture in Roman Gaul, the son of a high imperial official. He trained in law and rhetoric and by his early thirties held the governorship of Aemilia-Liguria, based at Milan, one of the western empire's most important cities. In 374 the sitting bishop of Milan died and the city split between Nicene Christians and Arians over a successor, a dispute serious enough that Ambrose, as the ranking civil official, went to the cathedral in person to keep order among the crowd. According to the tradition recorded by his early biographer Paulinus of Nola, a child's voice from the crowd cried out Ambrose for bishop, and the assembly took up the cry from both parties at once. Ambrose was still only a catechumen, not yet baptized, and tried to refuse, reportedly attempting to disqualify himself by demonstrating severity as a magistrate and even attempting to flee the city by night.
The emperor Valentinian I confirmed the election, and Ambrose was baptized, ordained through the full sequence of clerical grades, and consecrated bishop within about a week, an irregular haste that later canon law would not permit. He gave away his personal wealth to the church and the poor and set himself to learn theology as fast as he had to preach it, reading Greek writers including Origen, Basil of Caesarea and Athanasius to build the sermons an untrained bishop suddenly needed. He held the see for the rest of his life, until his death in Milan in 397, and became one of the most consequential churchmen of the fourth century Latin west.