In 1754 he started as violinist in the orchestra of the Archbishop of Breslau, Count Schaffgotsch; later he became concertmaster in the small town of Rudolstadt, and then in Plön under Duke Frederick Charles.
In Copenhagen he was immediately employed as a violinist in the Royal Chapel (Det Kongelige Kapel), became a sought after music teacher, including for the later King Christian VII.
The creation of a Concertmaster post in the Royal Chapel concurred with its expansion and transformation from a chamber music corps to an actual orchestra.
In Hartmann, the Chapel found a knowledgeable and experienced conductor, but also his masterful violin playing contributed greatly to the chamber concerts which at the beginning of Christian VII's reign were highly popular.
Balder's Death was first performed on 30 January 1779, and Hartmann's music, in which he had understood to strike "the solemn, melancholy tone", which the poet himself considered to be a main feature of most of his poems, won general acclaim.
Strikingly, Hartmann uses no less than two orchestras, the usual one in the pit and an additional band of 18 musicians back stage (including even three trombones and six horns, of which two "corni rustici", presumably a form of lur) .
The work contains two daring Valkyries terzets (of which the second one is accompanied by a trombone and a trio of bassoons), as well as (following his research on Icelandic music) an aria "In gusto Nationale d'Islandia" sung by Thor at the end of the second act.
The overture is drama in the high style and there are two entr'actes the first of which culminates in a "Ride of the Valkyries" and the second one in a storm announcing the dramatic events of the third act.
Alfred Einstein singles out this opera as an ″antecedent of the romantic opera″ which was ″written with Northern material seventy years before Richard Wagner″ and which belongs ″to the gloomy, mystical North″.
[1] The following year Johann Hartmann was equally successful with the music for The Fishermen (Fiskerne), whose subject allowed him to use lighter and festive colors.
The material is constantly varied and the first Act which describes the reactions to a shipwreck taking place off stage concludes with a Sextet in which the conflicting feelings of the characters are contrasted.
Act Three contains the famous romance Liden Gunver, which has become part of the Danish popular heritage, and culminates in a beautiful Quartet between the two couples.
The opera also contains the song Kong Christian stod ved højen mast, which was soon officially adopted in 1780 as the Danish Royal Anthem.