Findagrave.com has birth location as Newport, Campbell County, KY. However, 1900 census states that while the family was living in Newport City Ward 2 at that time, Mary and her older brother Benjamin were actually born in Ohio. No primary source of these births have been found yet.
Find A Grave, database and images (https://www.findagrave.com : accessed 29 September 2018), memorial page for Mary Clara Kettler Cook (10 Aug 1880–19 Jan 1929), Find A Grave Memorial no. 23738125, citing Saint Aloysius Cemetery, East Liverpool, Columbiana County, Ohio, USA ; Maintained by HALL LOVING GENEALOGY (contributor 46902586) .
"Ohio Deaths, 1908-1953," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:X6DQ-BLW : 9 March 2018), Mary Kettler in entry for Robert Cook, 07 Jan 1929; citing E. Liverpool, Columbiana, Ohio, reference fn 1303; FHL microfilm 1,991,721.
"United States Census, 1900," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:M9WK-PHK : accessed 30 September 2018), Mary Kettler in household of John Kettler, Magisterial District 1, Precinct C Newport city Ward 2, Campbell, Kentucky, United States; citing enumeration district (ED) 130, sheet 7A, family 131, NARA microfilm publication T623 (Washington, D.C.: National Archives and Records Administration, 1972.); FHL microfilm 1,240,512.