She was born in Auckland, New Zealand. She was educated by the Sisters of Mercy, and soon showed an exceptional musical talent. She adopted the vocation of a teacher of music, and took private classes in dancing, elocution, and singing.
She entered the Sisters of Mercy as a nun at the age of 28. While she was occupied in the regular work of her order in tending to the sick and needy, her outstanding skills as a music teacher became evident and she was encouraged to develop these talents.
She developed the already strong musical tradition of Saint Mary's College Auckland, with its orchestra, choirs, and individual tuition, and also conducted the Sisters’ Choir. From 1934, the college offered private tuition, and her reputation as a vocal coach flourished.
From 1950 on, honours and plaudits followed the remarkable success of many of her students, Major, Te Kanawa, Elisabeth Hellawell, Heather Begg, and Patricia Price and Judith Edwards to name only a few.
Sister Mary Leo initially began her teaching career as a violin teacher. She never received formal training in vocal technique. In the late 1930s, she heard a recording of Deanna Durbin. Sister Mary Leo was so taken by Durbin's natural beautiful tone, flexible technique, vocal range, repertoire that included both opera and light music, and her graceful poise, gowned in the Hollywood style, that Sister Mary Leo decided to devote her time to teaching singing. Consequently, there has been criticism that her pupils look and sound the same, as she strove to duplicate the Durbin sound.
At all times, Sister Mary Leo was a perfectionist with great vitality, who demanded the best from her students but required the same dedication of herself. Yet she remained modest and devoted to her life as a nun, giving of her talents to others.