Wilhelm was born to a Jewish family, the eighth of nine children of Ralph Merton (until 1856: Raphael Lyon Moses) and his wife Sara Amelie Cohen (daughter of Philipp Abraham Cohen), who immigrated to Frankfurt from England in 1837. On 5 November 1855, Ralph Moses and his family gained citizenship of the free city of Frankfurt. On 27 November, he was granted permission to call himself ‘Merton’ after claiming in his application for such (on 22 October 1856) that his brother Benjamin had already taken the family name ‘Merton’ in Manchester as the name ‘Moses’ was not suitable to be used as a surname.