He was educated at the cathedral school at Paderborn, of which he later became director.
He is venerated as a saint, but not officially canonised. He was also a prebendary in Aachen, court chaplain to Emperor Henry III and a canon in Goslar. In 1065 he became Bishop of Passau and began reforms of the clergy.
In 1070 he founded St. Nicholas' Abbey in Passau as a monastery of the Canons Regular and in 1083 Göttweig Abbey in Lower Austria, which in 1094 became a Benedictine monastery. He also reformed the existing monasteries of St. Florian, Kremsmünster Abbey, Melk and St. Pölten, and had stone churches built at all of them. In 1074 he announced the reforms of Pope Gregory VII, whom he supported in the subsequent Investiture Controversy.
In 1076 neither he nor Archbishop Gebhard of Salzburg took part in the Reichstag of Worms and, also like Gebhard, Altmann supported the counter-king Rudolf of Swabia. He was driven from Passau by supporters of Emperor Henry IV, who in 1077/1078 laid the town waste. In 1085 the Emperor deposed him as Bishop of Passau, after which he spent most of his time in the territory of the Babenbergers.
He died in Zeiselmauer in Lower Austria and is buried in the church of Göttweig Abbey. He is venerated as a saint, although never officially canonised. His feast day is 8 August.