Westcott was born as Jemima Isabel Casselman in Lauder, Manitoba, Canada on 10 January 1911. She was the sixth of eleven children born to parents Steven Casselman and Margaret (Tillie) Mahala McNaughton. She often worked on the family farm and later became a teacher.

In 1936, she married Reginald Westcott. The couple had five children. She lived in the settlement of Douglas, Manitoba for more than 25 years until a move to Brandon in 1960, where she contributed to church, school, and sporting life. Her husband died in 1965.

After retiring at the age of 65, she spent a year traveling in Australia. She lived independently in a condo until she was 106, after which she moved to a nursing home after some urging from her family. According to her granddaughter, around that time she had a paranormal experience. While sitting on her couch, she apparently heard some rattling in the kitchen, and after looking into the kitchen she saw her mother standing at the sink. Her mother turned to her and said “Mime, it’s time to go into a home.” The next day she started preparing to move into a nursing home.

Her older sister, Jean Janet Emily Holmes (1904–2012) died at 107, and another older sister, Aileen Mae Falconer (1909–2014) died at 105. Five of her siblings died in their 90s.

Westcott died in Brandon, Manitoba, Canada on 24 August 2022, at the age of 111 years, 226 days. She was survived by three children, 15 grandchildren and 28 great-grandchildren.