After winning prizes in competitions for younger musicians he obtained a position in the Vienna Philharmonic in 1978, at the age of eighteen. Between 1983 and 2008, he was the leader of the bass section and bass soloist of the Vienna Philharmonic Orchestra and of the Vienna State Opera. Since 1993 he has been a full professor at the Hochschule für Musik und darstellenden Kunst in Vienna.
He has a position as Guest Professor at the Mozarteum of Salzburg, and often plays as a guest soloist and chamber player. He has played on recordings released by Decca, Emi, Deutsche Gramophon, Sony, Philips, and EMI.
He is among the group of players who frequently play solo roles in projects organized by the Latvian violinist Gidon Kremer and thus participate frequently in Kremer's Lockenhaus Festival. Among his releases with Kremer is a chamber music version of compositions of the New Tango master Astor Piazzolla, on which Posch is the bass player and soloist.
Throughout his career, he has often performed at festivals around the globe, with distinguished colleagues, with many recordings documenting his collaborations.