He was probably born around the year 339 as son of the highest Roman administrative official in Trier in what is now Germany.  Contrary to legend, he did not come to Rome until after the death of his father.  Though Ambrosius was not yet baptized, Valerian appointed him governor of Upper Italy in 373 where he was to settle the dispute between the Arian and the Athanasian parties on the choice of a bishop.  In 374 he himself was made bishop and was baptized.  Legend has it that a child called out his name from the crowds quite audibly.
The bishops of the province, dreading the inevitable tumults of a popular election, begged Emperor Valentinian to appoint a successor by imperial edict; he, however, decided that the election must take place in the usual way. It devolved upon Ambrose, therefore, to maintain order in the city at this perilous juncture. Proceeding to the basilica in which the disunited clergy and people were assembled, he began a conciliatory discourse in the interest of peace and moderation, but was interrupted by a voice (according to legend, the voice of an infant) crying, "Ambrose, Bishop". The cry was instantly repeated by the entire assembly, and Ambrose, to his surprise and dismay, was unanimously pronounced elected. Quite apart from any supernatural intervention, he was the only logical candidate, known to the Catholics as a firm believer in the Nicene Creed, obnoxious to the Arians as one who had kept aloof of all theological controversies.