Sister Kathleen Therese was born Mary Cassidy in Portland, Maine, to parents John and Virginia (Ellis) Cassidy. She entered the Sisters of Notre Dame in 1960, took her first vows in 1963 and perpetual vows in 1968.
Sister Kathleen lives in Ipswich, MA. In 2022, she was awarded with a Cheverus Award medal by the Archdiocese of Boston, in recognition for a lifetime to service to the Church and God's people.