The third Catholic Parish in Janesville was organized in August 1951, when Reverend Joseph T. Strange was named pastor by the Most Reverend William P. O'Connor, bishop of Madison. 

The site of the new parish which was to be known as St. William was an 11-acre tract at North Oakhill and Mineral Point Avenues. The parish began serving about three hundred Catholic families in the northwest section of the city. Many of these families were known to Fr. Strange from his time as associate pastor at St. Patrick Parish from 1940-46.

The parish is named for St. William of Vercelli. Born in 1085, St. William founded the great monastary of Monte Vergine in Italy. He was known as a builder and a man of intense prayer and sacrifice. 

St. William Parish was officially founded in 1953 by Fr. Joseph P. Strange.  St. William Parish has had six pastors: Fr. Strange, Fr. Reardon, Fr. Klink, Fr. Auby, Fr. Dulli and now Fr. Leeser.