Mary Harris was born in Ithaca, New York, USA on 13 May 1911 to Italian immigrants, Joseph and Concetta (Delmonte) Petrillose. She was born premature, and the doctor said how she was too little and would not live.

In August 1937, she married James Richard Harris from North Lexington, North Carolina. They had two daughters, Connie Pitcher, and Dixie Billingsley. The couple ran a dry-cleaning business and dairy farm in New York. She worked as the bookkeeper both times. After leaving New York, they lived in Florida for 28 years. Eventually, they settled into retirement at the Ascension Living Alexian Village in Signal Mountain, Tennessee.

At the age of 77, she began painting. Her husband died at the age of 90, in September 2003. At the age of 105, she had five grandchildren and 12 great-grandchildren. On her 110th birthday, she received a letter signed by Pope Francis.

Harris currently lives at the Alexian Village in Signal Mountain, Tennessee, USA.