Catholicism in Delaware County, Ohio dates back as far as 1835 when German immigrant families began to settle in this area during their search for gainful employment. Between 1850 and 1860, the Big Four Railroad from Cleveland to Cincinnati was constructed by Irish Catholics. A large number of Catholic families moved to Delaware during this period because the area was centrally located. In 1854, a small crude wooden church was built on Hill Street, which is now University Avenue. In 1856, St. Mary Parish had its first resident pastor, Rev. Caspar Wiese.
The congregation soon outgrew the small church building. Eventually, in 1886, St. Mary parishioners built a large gothic style church using nearby stone and bricks. With a seating capacity of approximately 500 people, that building still stands today and serves its faithful with five weekend Masses. The parish has grown from 1,000 individual members in 1911 to 2,100 registered households today. The present St. Mary School building was constructed in 1925, with an addition and renovation completed in 2005. The former Sisters’ Convent, now utilized as Kavanagh Hall, was built in 1949.