A left-hander from Sankt Johann, Mandl first represented Austria at Davis Cup level in 1991, for a World Group qualifier against Great Britain in Manchester.

His loss in the doubles with Thomas Muster gave the tie to the home side and he got a consolation win in the reverse singles by beating Jeremy Bates.

He teamed up with Alex Antonitsch to beat Grant Connell and Glenn Michibata in the doubles, over five sets, which gave Austria a 2–1 lead.

On the ATP Tour, Mandl made it to as high as 186 in the world, twice reaching the second round at the Austrian Open Kitzbühel and once in Munich.

His only Grand Slam main draw appearance was in doubles, partnering Macedonian player Aleksandar Kitinov at the 1996 Wimbledon Championships.