After receiving the degree of Doctor of Jurisprudence (1803), he settled at Frankfort as an advocate, later being appointed, by Primate Karl Theodor Anton Maria von Dalberg, counsel of the municipal court (1806), counselor for the high schools and studies, and director of the grand-ducal lyceum (1812).
On the dissolution of the Grand Duchy of Frankfurt, Schlosser resigned his office, and in 1814 entered the Catholic Church with his wife Sophie (née Du Fay).
He was later one of the most determined champions of the rights of the Catholic community in Frankfort, and successfully advocated the civil equality of every Christian denomination.
Soon, however, he withdrew from public life, and after 1825 usually spent the winter in Frankfort, passing the summer at his country seat, Neuburg Abbey near Heidelberg.
As he was charitable, hospitable, and free from all denominational narrowness, and devoted himself wholeheartedly to scientific undertakings (e.g. the Monumenta Germaniae) besides possessing a fine artistic sense, his home soon became a centre for the leading spirits in literature, art, and science.