Kosegarten was born in Grevesmühlen, to pastor Bernhard Christian (1722–1803) and Sophia née Buttstädt in the Duchy of Mecklenburg-Schwerin.
He then served as the pastor of Altenkirchen on the island of Rügen, then part of Swedish Pomerania.
In 1777 he gave a speech on the birthday of King Gustaf III and wrote a hymn for the occasion.
[1] When the French occupied Pomerania he gave a speech on the birthday of Napoleon and was treated as a Bonapartist.
[1] He influenced the work of Philipp Otto Runge, Caspar David Friedrich, and the music of Franz Schubert.