Catholic Education in Tiffin, Ohio, dates back to the early mid-nineteenth century, when in 1847, St. Joseph School was started as a one-room log cabin with 20 students in its present location. A few years later, the first St. Mary School started in 1851, also as a one-room log cabin. This was located where Calvert High School currently stands. In 1858, St. Joseph School expanded to be a four-room school. In 1856, St. Mary Catholic Church moved to its present location. After a fire burned down the original St. Mary building, they purchased an old public school in 1864. It was located where the main Tiffin University classroom building currently stands.
In 1862, the Ursuline Sisters arrived from Germany. They took over the education at both schools and began the Ursuline Academy for high school girls. The academy was placed where the grassy lot next to Calvert High School is currently located.
Around 1923, at the request of Bishop Samuels Alphonsus Stritch, boys were admitted into the high school. It was at this time that the school took the name of Calvert Catholic High School. The first principal of Tiffin Catholic High School was Reverend Anthony J. Gallagher, but it was not until the 1925-1926 school year that the name Calvert was given to the school. This name comes from the Cecil Calvert family of Baltimore, Maryland. This family was known for promoting religious tolerance in the colony even while the Maryland colony was officially Catholic.