St. Mary was founded in 1842. The origin of the parish can be traced to the arrival of German immigrants to Summit Township, as Herman was called then. These Germans were primarily farmers. To attend Mass, they had to travel to St. Peter in Butler. In 1841, the community had grown to the point that they petitioned the bishop in Philadelphia for permission to form their own congregation. Permission was granted and they immediately began fund raising for a church. On June 22, 1842, the cornerstone of the church was laid and on July 6, 1845, the completed church was dedicated. Initially, the church was visited once a month from St. Philomena in Pittsburgh. In 1846, the first resident pastor was assigned to St. Mary Assumption.
The congregation grew steadily and by 1862 had outgrown the church. Rather than build a completely new building, the existing building was reconstructed. The new, larger building was dedicated on February 19, 1867. The church was renovated in 1891, 1897, 1904, 1917 and 1937.
By the beginning of the 1950's, it became obvious that a new church was needed. The final Mass in the original church was celebrated on August 19, 1853 and the razing of the building began on August 24, 1953. Until the new church was ready, Mass was celebrated in the auditorium of the parish school. Ground breaking for the new church took place on September 27, 1953. The cornerstone was laid on August 15, 1954 and the completed church was dedicated on September 18, 1955.
With the turn of the century, trends in the Diocese of Pittsburgh began to reveal a decline in Mass attendance and sacramental participation. At the same time, the number of priests available for parish ministry also began to decline.
To address these challenges, Bishop David Zubik announced on April 12, 2015 a new diocesan initiative, On Mission for The Church Alive!, a consultative strategic planning process designed to foster viable, sustainable and vibrant parishes. As part of this process, the Bishop, in consultation with the faithful, began to consider new models of parish life based on pastoral needs, financial and temporal resources and available clergy.
In 2018, following the period of consultation, parishes were grouped together and served by a single clergy team to eventually form one new parish.
During this transition period, St. Mary of the Assumption Parish remained an independent parish while sharing clergy and staff and eventually publishing a joint bulletin with the other parishes. This ended on January 4, 2021 when St. Mary of the Assumption Parish merged with St. Joseph Parish, Cabot and St. John Parish, Coylesville to form the new St. Francis of Assisi Parish. As part of the merger St. Mary of the Assumption Church remained open as part of the new parish.