Ordained in 776, he was named bishop of Salzburg in 785.
In 787 Arno went to Rome as an agent of Tassilo III, Duke of Bavaria, and when, after Tassilo's revolt and defeat, Charlemagne annexed Bavaria to his empire, the bishops of the country asked for Arno as primate. The emperor consented, and in 798 Pope Leo III gave Arno the pallium, with jurisdiction over Freising, Regensburg, Passau, Neuburg, and Laibach.
Arno held provincial synods at Riesbach in 799, 804, 807, and 810; he was also in charge of the missions in Carinthia and Pannonia, the latter province being then overrun by the Avars.
In 799, Leo III fled the violence of his enemies at Rome and was received in Germany by Charlemagne, who had Arno and Archbishop Hildebald of Cologne accompany the pope to Rome, there to investigate the charges made against him. Arno's opinion seems to have been unfavorable to Leo, but the matter was finally referred to Charlemagne, who accepted the pope's disavowal of guilt.
Arno also introduced the Carolingian Renaissance into Bavaria, where he assembled a library of 150 manuscripts and sent a number of students to Tours.