For over 30 years he was in the service of the
Ernestine branch of the Wettin family. A letter addressed to John Frederick I dating from 1536 is the earliest written proof of Gromann's service. The letter was signed as "stonemason" (). His teachers were
Konrad Krebs (also known as Kunz Krebs, died 1540 in Torgau) and Andreas Günter (died 1542 in Torgau), whose work he continued. While his first residence was located in
Weida, Gromann referred to
Gotha as his home in 1544. Six years later, in 1550, Gromann moved to
Weimar where he built his own house. In order to move back to Gotha, he sold it in 1563. A few years earlier, in 1553 Gromann had been allocated some acres of farmland in Gotha by John Frederick I. It was also the Elector of Saxony who appointed Gromann as master builder for life. Gromann first received special attention when he built the first
Protestant church, the chapel at
Schloss Hartenfels in Torgau, in the period from 1543 to 1544. The "Emperor's Room" () church was added skillfully into the medieval structure.
Luther sanctified the church in person upon completion. The model of the Torgau Castle Church formed the basis for the designs of the
castle chapels of Protestant princes in
Dresden,
Schwerin, Stettin (today
Szczecin),
Heidelberg and
Augustusburg. In 1552 Gromann applied a similar design to the chapel of the castle Grimmenstein, which was not preserved. Gromann supervised many construction sites in the
Ernestine duchies. In addition he built castles, town halls, churches, fortifications, roads, bridges and fountains. Nikolaus Gromann is credited with introducing of renaissance design to the Ernestine duchies. In his early work he expressed this by adding decorative renaissance ornaments to gothic buildings. In his late work he constructed buildings in the renaissance style, which are considered his masterpieces: the Town Hall in
Altenburg, the "new buildings" in
Weimar, the
Green Castle () and the French Building () of the
Veste Heldburg. Gromann implemented his design for the
bay windows in the living apartments of the elector on the north wing of the Castle Torgau and at the Französischer Bau of the Veste Heldburg. Today, the two bay windows are called the "Men's bay window" and "Women's bay window". His design of the Gera town hall could not be finished by himself. The project was continued by Nikolaus Theiner in 1573. ==Buildings==