Born into the family of a prosperous apothecary in Stockholm, Sweden, Wertmüller studied at the Stockholm Academy of Fine Arts with Pierre Hubert and Lorens Pasch the Younger. In 1772 Wertmüller moved to Paris and entered the studio of his cousin, Alexander Roslin. He also studied with court painter Joseph Marie Vien, traveling to Rome when Vien became director of the French Académie there in 1775. He remained in Rome until 1779, when he moved to Lyon. He returned to Paris in 1781, and soon became a member of both the Swedish Academy and French Académie. The King of Sweden, Gustave III, named Wertmüller “First Painter,” and commissioned him to paint Marie Antoinette and her children. Wertmüller also worked in Bordeaux, and, after the beginning of the French Revolution, in Madrid and Cadiz, traveling to the United States in 1794. The artist settled in Philadelphia, where he painted a number of portraits, including that of George Washington. He traveled to Europe to settle his financial affairs in 1796, then returned to Pennsylvania in 1800, where he soon marred Betsey Henderson, granddaughter of Swedish painter Gustavus Hesselius. Wertmüller became an American citizen in 1802, and purchased a farm in Delaware. He recorded his farming life in detail in his "Journal de la Terre situé à Naaman's Creek." While primarily known as a portrait painter and copyist, Wertmüller also painted mythological subjects. His Danaë Receiving the Shower of Gold was notorious for its nude central figure. He painted little during this time, but given financial difficulties charged admission for his Danaë.