According to the 1900 US Census in Manhattan NY; William was born Mar 1839 and emmigrated in 1853. He is married to Louise (maiden name of Walters - this from his son William Magnus Semnacher's 1880 Manhattan birth certificate).
A great big thank you to Eileen who was kind enough to give me some information on William M. Semnacher. She states that in the IGI, there is a birth & death listing for an unnamed daughter of Magnus Semnacher & Catharina Grimminger in Essingen, Jagstkreis, Wuerttemberg. She thought that Magnus might be William's brother. I was thinking more on the lines of his father. William M. Semnacher Sr named his son William Magnus Semnacher who was born 23 March 1880 and died 23 Oct 1943 in Elmore City, Elmore Co, Idaho. Can anyone confirm whether Magnus is William's brother or his father?
Appreciate any information given.