Upon the approval of Pope Pius XII in 1956, the State of Missouri was divided into four dioceses, and the Diocese of Jefferson City was formed. St. Peter Church was selected as the first Cathedral. The Cathedral of St. Peter served the founding bishop, the Most Rev. Joseph M. Marling, C.PP.S., for 12 years, from his installation in 1956 until Christmas Eve 1968 when the cathedral's jurisdiction was transferred to St. Joseph Parish. While it was the cathedral location, St. Peter Church witnessed history.

It was the site of many priestly ordinations, including that of the first priest of the diocese, Fr. John W. Buchanan, in 1957, who would return as the eleventh pastor of St. Peter in 1972. Fr. Anthony J. O'Connell, who was ordained here in 1963, would become the first priest of the diocese named a bishop. He was selected as the founding Bishop of Knoxville, Tennessee in 1988. Native sons of St. Peter who were ordained here in recent years include Fr. Donald J. Antweiler, pastor, St. Thomas the Apostle, St. Thomas, ordained May 26, 1973; and Fr. Michael J. Wilbers, diocesan Vicar General, and rector of St. Joseph Cathedral, ordained Feb. 26, 1972.

Other priests recently ordained at St. Peter and their year of ordination are: Fr. Philip M. Moriarty, 1980; Fr. John J. Schutty, 1983; Fr. Joseph Hoi, 1984; and Fr. Michael P. Murphy, 1989. Priests who were ordained elsewhere but said their First Mass at St. Peter include Bishop Charles Koester and Fr. John Henderson.

Another history making event for St. Peter took place in 1962 when it became the church of departure for the first group of diocesan missionaries to Peru commissioned by Bishop Marling. Still another historical event for St. Peter Church was being the site, on Holy Thursday, 1965, for the first Concelebrated Mass in the diocese. Monsignor Joseph A. Vogelweid, P.A., associated with St. Peter's for over 60 years, was the diocese's first Vicar General and the only rector of St. Peter Cathedral. He has become a legend in Jefferson City. Monsignor played a role in the foundation of the Carmelite Monastery, St. Joseph's Home for the Aged, and the establishment of the city wide Veteran's Day Mass at St. Peter. Upon his retirement in 1969, his assistant for 17 hears, Fr. Norman J. Ahrens, became the tenth pastor. That same year, Fr. Gerry Kaimann, arrived as the first in a long line of transitional deacons.