As Princeton's enrollment surged following the Civil War, the College's long-standing shortage in dormitory space intensified, with a majority of the students living and dining off campus. To address this problem, President McCosh built five new dormitories during his administration. The first of these, and the only one that does not exist today, was Reunion Hall (Architectural Catalog Entry).

Reunion Hall was commissioned by the Trustees in June 1869. Its name derived from the 1869 "reunion" of the two branches of the Presbyterian Church, which had split into Old and New schools a century earlier. The cornerstone was laid in June 1870, and the ceremony was attended by a committee of 20 church officials appointed by the General Assembly of the Presbyterian Church.

George B. Post, who had designed both the new Bonner-Marquand Gymnasium and Dickinson Hall, was selected to design the new dormitory. He produced a five-story building with 70 rooms that mixed High Victorian Gothic with the Second Empire elements, notably the elaborate and distinctive mansard roof. East and West Colleges acquired similar mansard roofs in the 1870s as well.