William Thomas Mahoney was born April 19, 1916, in Chattanooga, Tennessee, and received his elementary education at Notre Dame Grade School in Chattanooga and at St. Edmund's Grade School in Oak Park, Illinois, and his secondary education at Fenwick High School in Oak Park and Notre Dame High School in Chattanooga. After graduating magna cum laude with a degree in philosophy from the University of Notre Dame in South Bend Indiana, he entered the Dominican novitiate at St. Rose Priory in Springfield Kentucky, where he was assigned Bertrand as his religious name. First profession followed on August 16, 1939, and he continued with philosophical and theological courses at the Dominican House of Studies in River Forest, Illinois, where he was ordained to the priesthood on March 23, 1945.

In May 1946, Father Mahoney completed the requirements for the Lectorate in Theology and was then sent to Rome for a Doctorate in Philosophy from the Angelicum. In 1948, he returned to the States to teach philosophy in River Forest; in 1951, he was assigned to the Philosophy Department at the Angelicum where he served as a professor until 1958, except for the academic year of 1956-1957 when he taught once again in River Forest. From 1958 until 1970, Father Mahoney taught philosophy in the River Forest Studium while serving as Master of Studies, Academic Dean for Aquinas Institute, and as Subprior for the Dominican community. In 1970, he was assigned to St. Rose Priory in Dubuque, Iowa, to teach philosophy at Aquinas Institute after the School of Philosophy closed in River Forest.

During the summer of 1970, while celebrating his Silver Jubilee with his family in Chattanooga, Father Mahoney developed a severe lower intestinal disorder that led to colostomy surgery. Soon after that, a circulatory ailment and diabetic condition led to amputation of his right leg. While undergoing therapy at the Alexian Brothers Rehabilitation Center in Signal Mountain, Tennessee, the same condition affected his left leg, and a second amputation was required.

Although confined to a wheelchair, Father Mahoney returned to St. Rose Priory in 1972 and continued to serve the community to the best of his ability as Adjunct Professor of the History of Philosophy, Assistant to the President of Aquinas Institute, Subprior, and Assistant Business Manager, in addition to providing editorial assistance for Dominican Life (now U.S. Dominican) and Cross and Crown (now Spirituality Today). He died suddenly, of apparent heart failure, shortly after lunch on December 16, 1980. Following liturgical services at St. Rose Priory in Dubuque, his body was brought to Chicago for burial in the Dominican plot at All Saints Cemetery, Des Plaines, Illinois, on December 19.