sixtus was a son of johann tannberg of aurolzmünster and ursula von rohr, a sister of archbishop bernhard von rohr. in 1442 he was admitted in freising as an exspektant and in 1456 he joined the local cathedral chapter. he studied at padua for almost eight years and became a doctor of both laws. in 1458 he became provost in isen in upper bavaria and in 1466 pastor of laufen. allegedly he was also chancellor of bishop john iv tulbeck of freising and canon at salzburg.