In 1816 Thomsen at age 27 was appointed to succeed the retiring Rasmus Nyerup as Secretary of the "Kongelige Commission for Oldsagers Opbevaring" ("Royal Commission for the Preservation of Antiquities"), which had been founded in 1807. The post was unsalaried; Thomsen had independent means. At his appointment Bishop Münter said that he was an "amateur with a great range of accomplishments." Between 1816 and 1819 he reorganized the commission's collection of antiquities. In 1819 he opened the first Museum of Northern Antiquities, in Copenhagen, in a former monastery, to house the collections. It later became the National Museum.