He is best known for his role as vocalist and one of the founders of the band At Vance, his own band Hartmann and his guest appearances on albums of several prominent metal bands, including Freedom Call, Edguy, Rhapsody, Genius rock opera, Iron Mask, Aina's metal opera Days of Rising Doom and the metal opera Avantasia where he participated as a vocalist in 4 albums and also as a guitarist of the live and studio line up.

Hartmann also sang in Italian band Empty Tremor, which was a support act for Dream Theater during a 2004 tour in Italy.

After the release of Avantasia's 2010 double album "The Wicked Symphony/Angel of Babylon" and a second world tour in December 2010 the live DVD The Flying Opera – Around the world in 20 days, recorded during the first tour in 2008, was released in early 2011 and climbed up many European charts to No 1.

Ian Gillan, Jimi Jamison, Steve Lukather, Chris Thompson and Robin Beck) on vocals and guitars.

The recordings for the Hartmann album "Balance" were completed in April 2012 and released on the label Avenue of the Allies.