The German Congregation played a significant role in the history and development of St. Mary's Church. Following WWI, many German-speaking Catholics immigrated to Canada, and settled in Kitchener and the surrounding area. Many of these immigrants were from the German speaking regions of the former Austro-Hungarian Empire. In 1928 Father Albert Zinger, C.R. celebrated the first Mass in German at St. Mary's in the lower church. A second wave of German speaking immigrants arrived after WWII. The majority were forced to flee their homes in the German-speaking regions of central and southeastern Europe, such as Hungary, Romania and Yugoslavia. During this time there were no German Masses at St. Mary's. Some dedicated parishioners approached Bishop Joseph Francis Ryan with the goal of establishing a German Congregation at St. Mary's Church. Father Leo Siess, C.R. was appointed Pastor of St. Mary's German Congregation and in 1948, Mass was again celebrated in German in the lower church. The parishioners found a place where they could preserve their German heritage through prayer and song.
In 1965 the Pallottines were officially appointed as pastors of St. Mary's German Congregation, beginning with Father Willi Kröll, S.A.C. who celebrated his first Mass on March 7, 1965. This was a very important year for the German Congregation: the German Ladies Group (Frauengruppe) was founded; the German Congregation held its first parish picnic; the German Youth Group was founded and the first Christmas Bazaar (Weihnachtsmarkt) was organized by the German Ladies Group. The first German Parish Council was established in 1966. On September 10, 1967, German Mass was celebrated for the first time in the upper church. In 1969 the German Congregation celebrated its first annual Seniors Day Celebration, and in 1970 the German Men's Group was founded. The Marian Choir (Marienchor) held its first practice in 1970. A children's choir was also formed. The German Congregation even had its own radio program on one of the local stations. Kontakt was a program of prayer, music and discussion heard each Sunday evening.
The Pallottines continued to serve St. Mary's German Congregation until 2013:
Fr. Willi Kröll - 1965 - 1969
Fr. Karl Trabold - 1969
Fr. Josef Schmelz - 1969 - 1972
Fr. Wolfgang Weiss - 1972 - 1978
Fr. Wilfrid Systermans - 1978 - 1985
Fr. Alois Mutert - 1985 - 1986
Fr. Gottfried Seifert - 1986 - 1990
Fr. Theodor Gerharz - 1990 - 1995
Fr. Gregory Roetker - 1995 - 2002
Fr. Martin (Jacek) Mikulski - 2002 - 2013
Following the departure of the Pallottines, Father George Nowak, C.R., and Father Paul Szymanowski, C.R., continued to celebrate the Mass in German from 2013 until 2018. With a congregation that was aging, and decreasing in number, it was decided in 2019 that the time had come to merge the German Congregation with the English Congregation. Although the numbers have diminished, the German Congregation is a group of parishioners who have stayed together as "one family", united through their German heritage, including language, and their strong faith in God.