He immigrated to the Thirteen Colonies in 1773 aboard the Crawford; a fellow passenger was John Contrad Trevitz, who would also go on to become a teacher and fraktur artist.
By 1793 he had acquired land in Conewago Township, Dauphin County; during this period he began to produce fraktur, primarily for his neighbors.
He taught school, most likely at the institution operated by Hoffer's Brethren Meeting; even so, he produced mainly baptismal certificates during his career.
Mertel's paintings bear the influence of other fraktur artists, including Johann Henrich Otto and Johann Jacob Friedrich Krebs, who was a neighbor; nevertheless they are marked with the stamp of individuality, containing images of castles, fish, mermaids, lions, unicorns, the Princess of Brunswick, and flowers of many varieties.
He died in Conewago Township; a list of effects at his death included a box of pictures, an inkstand, and a musical instrument.