Weir was born as Mary Elizabeth Schenkel in New Trenton, Franklin County, Indiana, USA, on 16 February 1909. Her parents were Charles George Schenkel (1868–1948) and Mary Katherine Faber (1873–1955), both of German descent. She had eight siblings. Her family were members of the former Holy Guardian Angels Parish in Cedar Grove. During the Great Flood of 1913, her family home was safe since it was on a hill.

In 1928, she married Lawrence Paul Weir (1904–1963). They moved to Indianapolis after the marriage. The couple had four children between 1936 and 1946. In 1940, they moved from Greenfield to Fortville. In 1946, they moved to Ingalls where she worked in the cafeteria at old Ingalls school. She retired after 23 years.

Weir passed away in Fortville, Indiana, USA, on 19 November 2019, at the age of 110 years, 276 days. She was survived by her four children, 9 grandchildren, 16 great-grandchildren and 11 great-great grandchildren.