In 1870 Father Camillius Maes of Saint Peter Parish erected Saint Mary's School on this lot, adjacent to the church. When the grammar school opened in that year, it was the first Catholic school in the city and in Macomb County. Three nuns from the Immaculate Heart of Mary Convent in Monroe were chosen to teach the students, eighty in the first year. The school was a two-room, two-story wooden structure. The boys were taught in one room and the girls in the other. All received instruction in basic elementary education subjects and chuch doctrine. By 1882 Saint Mary's annual enrollment numbered about 170 students. The first class — all girls — was graduated in 1885. The boys had their first graduation in 1909. The sisters of the Immaculate Heart of Mary taught at the school until 1986, when lay teachers look over the instruction.
When the original portion of the present Saint Mary's School opened in 1889, the Mount Clemens Monitor called it "one of the most substantial and best finished buildings in Mount Clemens." Built in the Second Empire style, it was a square, towered, three-story building with classrooms and offices on the first two floors and an auditorium on the third floor. The third story burned in 1957 when a fire at Saint Peter's Church caused the steeple to topple onto the school roof. Fortunately only the third floor auditorium was lost. The school's four distinct stages of construction typify the evolution of schools as they grew to serve an ever-increasing population. In 1903 an addition was erected on the back of the original building. A high school wing was built in 1911, and an addition to it was constructed in 1921