In 1855, Calhoun County was established. The Illinois Central Railroad was built through the county  in 1870, and by 1875 Pomeroy was a growing community. Priests from Fort Dodge attended to the spiritual needs of the community and first celebrated Mass in the John Knight home. As the congregation grew, services were then held in the Pomeroy school. St. Mary's Church was built around 1881 with John Knight, Patrick Clancy, Maurice Clancy, J.H. Hogan, and William Taylor serving on the building committee.

The Pomeroy cyclone completely destroyed this Catholic Church and all of its records on July 7, 1893. The debris was scattered two hundred feet or more, but the tabernacle was left standing and the chalice, communion cup and fixtures were undisturbed. Tha altar stone and a chair donated by John Knight were also not destroyed. Parish members worked diligently clearing away debris and made plans for rebuilding the church. Again Patrick Clancy was chairman of the building committee. Father M.C. Daly, pastor of St. Thomas parish, Manson, had Pomeroy assigned to him as a mission parish. During good weather he celebrated Mass outside on an improvised altar with planks laid across tiles for pews and in inclement weather services were held in homes until the new church was built.

It is unclear exactly when the cemetery property was acquired; however, it was thought to be about 1885. John L. Dalton donated a heifer which was raffled off and won by Michael Clancy, who in turn raffled it off again. With this sum of money they bought the present site of St. Mary's cemetery. For the donor's combined efforts, they were each given a lot in the center of the plot where Mr. and Mrs. John L. Dalton and Mr. and Mrs. Michael Clancy are buried.

Father John McAuliffe succeeded Father Daly in Manson in1902 and drove a team of horses to Pomeroy to celebrate Mass and teach catechism twice a month. He often picked up Oliver and Ernest Baker on his way to Pomeroy, and they in turn would care for his horses.

In 1909, Father Edward J. Neppel, the first resident priest, lived in various homes until the rectory was built in 1910. St. Joseph's Parish of Palmer became a mission of Pomeroy at this time. The parochial school in Pomeroy was built north of the church at a cost of $12,000. It opened in the fall of 1915 with Sr. Theodotia and Sr. Alexander, Franciscan Sisters of the Holy Family, as the first teachers. Margaret McAlpin was the first child to enter. The school was a combination of boarding school, day school, and home for the sisters. At one time approximately 60 boarders from the towns of Palmer, Knoke, Jolley, and Manson. The students stayed from Monday through Friday. From 1928 to 1932 eleven grades were taught at the school.

Father Joseph M. Neppel succeeded his older brother as pastor in 1923.

In 1934, Father Richard Graf became pastor. Because of the Depression, children often brought food to pay for their board, and tuition was taken only from those children outside of St. Mary's and St. Joseph's parishes.

Father James Kane arrived as the new pastor in 1938 and remained until 1944 when Fr. Norbert J. Boes came to serve. He remained in the area for ten years. Following his departure, the following priests served Pomeroy and Palmer in rapid succession: Fr. Raymond Calkins, 1954-1955; Fr. Louis Greving, 1955-1956; Fr. Thomas Lawless, 1956-1958; Fr. G.A. O'Rouke, 1958-1959, and Fr. L.J. Eisenbacher, 1959-1961 and Father Alfred E. McCoy (1961-1965).

During the tenure of Fr. Charles Yetmar,1965-1971, the history of the parish again took on a new sense of direction. In 1967 the school was closed due to declining enrollment but continued to be used for weekly CCD classes andstorage. In the spring of 1966 the idea of a new church was instituted.A building committee consisting of Adolph and Ardyce Lenz, Leo Clancy, Don Hammen, Urban Korte, Loren Gerdes, Wayne Hansen, James Murphy, Jr., Hubert Lenz, Adolph Peiffer, Glen Holland, Dennis Lenz, Cliff Meyer, Bernard Gugglsbert, Jerome Wagner, and Arnold Streit was formed to tackle all aspects of such an undertaking. When Adolph Peiffer offered avery generous gift in 1967, a drive was begun and enough money was pledged to make the dream a reality. On August 15, 1969, on the Feast of the Assumption, the first spade of dirt was turned. One year later the cornerstone was placed in the newly constructed church. The new church, with an adjoining parish hall, was placed just east of the old one. Reminders of the old church were the original large crucifix, the altar stone, and the Stations of the Cross which were hand painted by Sara Baker. The new church was officially dedicated on August 23, 1970.

Father Lawrence McCarty (1971-1975), became pastor in 1971. It was during thistime that the Cursillo Movement became very strong in St. Mary's congregation and many meetings were held regularly in the rectory and parish hall. On January 20, 1973 the bell of St. Mary's again began ringing after many years of silence. Deterioration of the tower of the old church had forced the removal of the bell and tower some years before. The free-standing tower, made of the same brick used in the new church, was completely electrified but could also be operated manually. Several years later a brick encasement was built in the front of the church to indicate times of Masses and confessions.

Fr. William Wingert served the parished from 1975 to 1977 and was succeeded by Fr. Robert Keefe. Father Keefe taught his parishioners what it was like to struggle with illness. He demonstrated great perserverance and strong faith through many surgeries and finally the amputation of a leg. He left in 1981 for a parish with a handicapped accessible rectory.

Fr. Robert Leiting was then the pastor until 1982 and was followed by Fr. Edward Young until 1984. Fr. Peter Fransco served the parish from 1984 until 1991.

In the early summer of 1991 Fr. Richard Macke came to Pomeroy. Shortly thereafter the school building was demolished by fire and a double garage with storage space was constructed.