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.

Measuring 125 feet long and 25 feet wide, the building featured gray stone walls offset by red brick quoins. Seen for the first time on the campus in Reunion Hall, this deliberate contrasting of color was a notable characteristic of other High Victorian Gothic structures built in the McCosh era.

Located between West College and Geological Hall, Reunion Hall was built on the former location of the Joseph Henry House, which was moved to the eastern side of the campus. Deliberately off-line in its placement, Reunion negated the symmetrical plan for the College envisaged by Joseph Henry in the 1830s.

For students, Reunion was problematic from the beginning. Lit by gas and heated by steam, these systems were intended to improve on the old kerosene lanterns and coal fires of the other dormitories. But the steam heat proved unreliable, causing residents much misery. And then there was the political geography. Advising students to shun the new dormitory, one undergraduate wrote in 1871, "Every bit of fun you have [in Reunion] will be distinctly audible at the President's House."

Fire escapes were added to the exterior of Reunion in 1882 and later a low arch constructed to connect Reunion to West College.

Used variously for dormitory rooms, administrative offices, and the headquarters for student publications (before the acquisition of 48 University Place), Reunion Hall did not age well. By 1958, its top two floors had been condemned as a fire hazard. It was finally demolished in 1965.