1900 United States Federal Census about Wm M Barnum. Name: Wm M Barnum. Home in 1900: Louisiana, Pike, Missouri; Age: 34; Estimated Birth Year: abt 1866; Birthplace: Missouri; Relationship to head-of-house: Head; Spouse's Name: Nellie B; Race: White; Household Members: Wm M Barnum, 34; Nellie B Barnum, 33; Maggie M Barnum, 18; William C Barnum, 14; Joseph A Barnum, 10; Louis Barnum, 8; Mary Barnum, 3.
1910 United States Federal Census about William M Barnum. Name: William M Barnum; Age in 1910: 53; Estimated Birth Year: abt 1857; Birthplace: Missouri; Relation to Head of House: Head; Father's Birth Place: Ohio; Mother's Birth Place: Ohio; Spouse's Name: Nellie; Home in 1910: Louisiana Ward 2, Pike, Missouri; Marital Status: Married; Race: White; Gender: Male. Household Members: William M Barnum, 53; Nellie Barnum, 52; Joseph A Barnum, 21; Louis Barnum, 18; Mary Barnum, 13.
1920 United States Federal Census about William M Barnum. Name: William M Barnum; Home in 1920: Louisiana Ward 2, Pike, Missouri; Age: 62 years; Estimated Birth Year: abt 1858; Birthplace: Missouri; Relation to Head of House: Head; Spouse's Name: Nellie M; Father's Birth Place: Ohio; Mother's Birth Place: Ohio; Marital Status: Married; Race: White; Sex: Male; Home owned: Own; Able to read: Yes; Able to Write: Yes. Household Members: William M Barnum, 62; Nellie M Barnum, 61; William C Barnum, 32.
1930 United States Federal Census about William Barnum. Name: William Barnum; Home in 1930: Buffalo, Pike, Missouri; Age: 73; Estimated Birth Year: abt 1857; Birthplace: Missouri; Relation to Head of House: Head; Spouse's Name: Nellie B; Race: White. Household Members: William Barnum, 73; Nellie B Barnum, 72.
According to his death certificate he was a retired dry goods merchant.