Mercy High School was opened in the fall of 1924 at the corner of 81st & Prairie Avenue as five parish schools either had their enrollments altered or closed their doors to form a centrally-located school on Chicago's south side. Those schools included St. Elizabeth (which stayed open for African-American students), St. Ita, St. Patrick on the Southeast side, St. James, and St. Gabriel. The Sisters of Mercy were in charge of the all-girls' school (hence, the name Mercy in the title), and occupied one square city block with their building, which cost one million dollars to build between 1924-26.

A total of 712 students were enrolled that first school year in either academic or commercial classes. The first graduates left the school in spring of 1926 as a class of 126 strong. Although the commercial program was discontinued in 1932, the school grew to a high of 1,187 students in the fall of 1943 and stayed over 1,000 for well over two decades. African-Americans were accepted beginning in 1954 as the racial makeup of the south side began to change.

Mercy experienced a decline in enrollment, fewer religious teachers from the Sisters of Mercy community, and an increasing financial deficit as 1970 came along, and discussions began to take place about the school's future. After a considerable amount of time, it was decided to merge Mercy and Loretto Academy from the Woodlawn neighborhood in the fall of 1972 in the Mercy building, at the urging of Rev. Robert Clark, who served as the superintendent of schools in the Archdiocese of Chicago at that time.

The new school was called Unity and remained open until 1980 when it was part of a merger with St. Thomas the Apostle, Visitation, and Aquinas Dominican to form the VAUT Corporate System, which opened two new schools, Unity Catholic and Aquinas Catholic. You can read more about each school by clicking on their names.