Theissen was born in Breckenridge, Minnesota, USA, on 28 December 1907. Her parents were Clemens Schaefer (1865–1936) and Mary Leinen (1872–1968), both German immigrants. She had four siblings: Clemens (1893–1895), Clarence John (1894–1910), Gertrude Alice Hintgen (1901–1976) and Rita Elizabeth Brenner (1909–1996). She graduated from Breckenridge High School and continued her education at the State School of Science in Wahpeton, North Dakota.

In 1929, she married Elmer Theissen (1905–1991), a pharmacist. The couple eventually moved to Fort Madison, Iowa, where they bought the Ford Hopkins Drug Store and renamed it Theissen Rexall Drug.

Her husband passed away on 15 September 1991. In 2004, she moved to Merrill, Wisconsin to live with her daughter. She was a member of St. Francis Catholic Church in Merrill.

Theissen passed away in Merrill, Wisconsin, USA, on 23 May 2018. She was survived by three daughters, 13 grandchildren, 37 great-grandchildren and 16 great-great-grandchildren.