William J. Leonard was born on April 10, 1908 in Dorchester, Massachusetts, to William J. and Catherine V. (Smith) Leonard. Leonard was the oldest of five children, including two sisters, – Catherine and Eleanor – and two brothers – Francis and Robert.

After attending elementary and grammar school in Dorchester, Leonard attended Boston College High School, where he graduated in 1925. Leonard joined the Society of Jesus the summer after graduating from high school. He took his first vows after a two-year novitiate in 1927, followed by a two-year Juniorate at Shadowbrook. Leonard's education continued with courses in philosophy at Weston College starting in 1929. In 1931, Leonard received his AB Degree from Boston College, followed by his Master's Degree in 1932. The Jesuit provincial assigned Leonard to his first teaching position in 1932, with the English Department at the College of Holy Cross. While at Holy Cross, Leonard served as an instructor in English and "prefect" of student Masses. On June 20, 1937, Leonard was ordained a priest, and a year later he earned his Licentiate in Sacred Theology. Leonard completed his final period of Jesuit formation – the tertianship – at St. Robert's Hall in Pomfret, Connecticut. Leonard took his final vows in 1942.

Leonard joined the Boston College Jesuit Community in 1939, serving as an instructor in English literature and as faculty moderator to The Stylus (a student literary journal). After a hiatus from teaching, due to his military chaplaincy, Leonard returned to the English Department at Boston College in 1946, where he continued to teach until 1952. In 1953, Leonard was transferred to the Theology Department full time, where he continued to teach until his retirement from teaching in 1973. During his tenure with the Theology Department, he served as chairman from 1965 to 1969 and presided over changes to the theology curriculum. From 1969 to 1973, Leonard also taught theology at the Pope John XXIII National Seminary in Weston, Massachusetts. Leonard returned to the classroom for the 1997-1998 academic school year to teach one of the Capstone Seminars for Boston College seniors.

Leonard's military service as a chaplain interrupted his teaching at Boston College from 1944 to 1946. Although Leonard registered for the draft in October of 1940, the Provincial only accepted his offer to serve as a military chaplain once student enrollment at Boston College began to decline due to the armed services' manpower requirements. Admitting to having no attraction to military life, Leonard did recognize that he had a role to play as a chaplain.

Leonard's military career began with six weeks of training, primarily in military organization and customs, at Harvard University. Upon graduating from chaplaincy school, Leonard transferred to Camp Livingston, Louisiana, for three more months of training. Leonard was transferred to Camp Beale in California for three weeks, followed by a three week boat journey to New Guinea. In New Guinea, Leonard joined the 9th Ordnance Battalion, which was in charge of the procurement, maintenance, and issuance of weapons. Leonard spent six months in New Guinea, the highlight of which was the construction of the Finschhafen Chapel. He was sent to Manila in April of 1945. At the conclusion of the war, the United States government discharged Leonard and he returned to his teaching position at Boston College.

Leonard was highly active – in official and unofficial capacities – in the liturgical movement that was taking place in the United States. He became involved with the National Liturgical Conference and participated in the Conference-sponsored Liturgical Weeks from the 1940s through the 1960s. Moreover, he served on the Board of Directors of the National Liturgical Conference from 1952-1958, as the Secretary from 1958-1963, and on the Advisory Council from 1963-1969. At Boston College, Leonard founded and directed the Social Worship Program, which ran from 1947-1952. Within the Archdiocese of Boston, Leonard served on committees and conducted numerous programs in the theology of public worship, including Mass demonstrations and talks on the dialogue mass. Leonard's activity with the liturgical movement also extended across the Atlantic. He served as a delegate to the First International Congress on Pastoral Liturgy in Assisi-Rome in 1956. In 1963 and 1964, Leonard served as a Liturgical Advisor to the Second Vatican Council.

With the liturgical changes that resulted from the Second Vatican Council, Leonard continued his involvement with the liturgical movement, particularly in the capacity of preserving and remembering the changes within the Catholic liturgy over the course of the twentieth century. Encouraged by Philip McNiff, his friend and director of the Boston Public Library, Leonard initiated a project to collect and preserve the objects of the Catholic experience from 1925-1975. Leonard envisioned that the "Liturgy and Life Collection" would encompass an understanding of the liturgy that included church ceremonies and the Sunday mass, but moved beyond to incorporate those activities and places where parishioner's manifested faith, hope, and charity. Leonard served as curator of the Liturgy and Life Collection at the John J. Burns Library from 1978 until his death in 2000.

His liturgical involvement also extended to the planning and direction of the Liturgical Consultation at Boston College in 1983. The consultation brought together both "pioneers" of the liturgical movement, and younger people involved in the continued emphasis upon liturgy in the Catholic experience.

Leonard's religious activities extended beyond his involvement in the liturgical movement. At the Archdiocesan level, Leonard was highly involved in spiritual development. He conducted retreats, assumed parish responsibilities during summer months, directed discussion groups including the Needham Bible Groups and the Holy Spirit Guild, and preached at numerous parishes and events throughout his life. Moreover, he served as Religious Superior of the Jesuit weekly America in New York from 1977-1980. His duties at America focused on the spiritual lives, health, and material needs of the community.

Throughout his life, Leonard remained a prolific writer publishing numerous book reviews, articles, and books. His articles primarily focused on the experience of the liturgy. Leonard's monographs also reflected his interest in liturgical reform and lay involvement in the liturgy. He authored The New Instruction for American Pastors on Sacred Music and Liturgy (1959), and edited a collection of essays in honor of Gerald Ellard, S.J. (1963). He co-authored, with Leon A. McNeill, New Horizons in Catholic Worship: Liturgical Renewal in the Light of the Constitution on the Sacred Liturgy (1964) and New Horizons in Christian Living: The Overflow of the Liturgy into Personal, Domestic, and Social Life (1965). In the mid-1990s, Leonard published two monographs that focused upon his life, The Letter Carrier: The Autobiography of William J. Leonard, S.J., (1993), and Where Thousands Fell (1995).

As is evidenced by his activities as a teacher, theologian, chaplain, priest, lecturer, and writer, Leonard was continually on the go and averse to slowing down. His desire to remain active and contributing religiously or academically was evidenced in his disagreement over mandatory retirement for religious faculty members at the age of sixty-five. In 1973, at the age of sixty-five, Leonard questioned "Who said I was finished?" His life was one of continued activity, and a challenge to avoid "restlessness, the pernicious demon."

Leonard died on February 12, 2000, in Weston, Massachusetts.