Under the leadership of Reverend Joseph T. Strange, appointed pastor by the Most Reverend William P. O’Connor, first bishop of the Madison Diocese, St. William Parish was established in August, 1951 to serve approximately 300 families. Site of the new parish was an 11-acre tract at North Ravine and North Arch Streets.

In June, 1952, following a triple wing design, ground was broken for construction of the first wing, the basement portion of the gymnasium. The basement would serve as temporary church quarters with a seating capacity of 600, as well as an area for religious instruction and parish social functions. At the south end of the nave would be a sanctuary and sacristy and at the north end, a mother’s room, storage and rest room facilities, and a kitchen. Included in the initial building program was the construction of a seven room rectory.

On November 16, 1952, Monsignor Joseph Delany, pastor of St. Patrick’s Church, officiated at the laying of the cornerstone and on November 30th, the first Sunday of Advent, a congregation of about 400 families heard mass in their new church.

Construction of the first wing of the school – seven classrooms, a large foyer and an office – was begun in May of 1953. By fall, four rooms were finished completely and the unoccupied space was adapted for living quarters of the Sisters of Mercy who would staff the school. With Sister Mary Giles as principal, school opened September 14th for 136 pupils in grades 1-4. In 1956, a convent was constructed on Arch Street, and the first graduates of the school received their diplomas in June, 1957.

Additional construction 1958 included a third wing of three classrooms, a foyer, a new kitchen, an all-purpose room (dining room) and a gymnasium. Later, in 1961, two classrooms were added to the third wing and a section of the foyer was converted into a library and new office, freeing old office space for a teachers’ lounge. A four-room addition enlarged the rectory.