Mary Catherine Harnan was born in Colorado Springs, Colorado on January 3, 1904. She was one of seven children born to John Harnan and Catherine Hogan Harnan. In 1909, her family moved to Seattle, Washington, where her father practiced law. She received her elementary and secondary education from Holy Names Academy in Seattle.
After graduating from Holy Names Normal School in 1924, Mary entered the novitiate of the Sisters of the Holy Names of Jesus and Mary in Oswego, Oregon. She received the name Sister Loyola Mary and made her temporary vows on August 5, 1926. She made her perpetual vows on August 5, 1929. Between 1926 and 1932, Sister Loyola Mary taught in Seattle, Spokane, Portland, and The Dalles.
Sister Loyola Mary was an accomplished artist and calligrapher. She pursued special studies in art and calligraphy at the Schaffer School of San Francisco, University of Washington, California School of Fine Arts, and Chicago Art Institute. She also studied under the tutelage of famed calligrapher Lloyd Reynolds. In 1933, she joined the faculty at Marylhurst College, where she taught art structure, posters, lettering, costume design, drawing and painting, applied design, fashion illustration, interior design, commercial design, and calligraphy. She retired in 1975 but continued to work part time until 1977 and served the college as a consultant until 1985.
Sister Loyola Mary passed away on November 30, 1992 in Lake Oswego, Oregon and was interred at the Marylhurst Convent Cemetery.