Of the surviving McCosh-era buildings, the Chancellor Green Library has best stood the test of time and changing taste. This may seem counter-intuitive, as 20th-century Princeton has generally frowned on the High Victorian Gothic style, and Chancellor Green was perhaps the most pronounced example of the style erected on the campus.

But it was also the most successful. A symmetrical building featuring strong geometric forms, rich ornamentation, and a marvelous octagonal rotunda (the ceiling in Chancellor Green Library is one of the most striking structures in Princeton), the library is characterized by its own grace and integrity.

Designed by William A. Potter, Chancellor Green Library was commissioned in the spring of 1871 and was underwritten by McCosh's great patron, John C. Green. The building was named for Green's brother, Henry W. Green, class of 1820, a trustee of the College and Chancellor of New Jersey. The foundation was laid by November 1871, and the walls finished in time for Commencement the following spring. It was dedicated in May 1873 and its costs exceeded $100,000.

The first purpose-built library on the campus, Chancellor Green Library reflects one of McCosh's top academic priorities. When McCosh arrived in Princeton in 1868, he was appalled at the state of the College's library, which was open only one hour per week. If the College were to improve its academic stature, it clearly needed a better library.

In December 1870, he sounded out the Trustees about raising funds to construct a new library. It was the fourth building that McCosh initiated and, as with the others, it addressed a critical gap in the College's physical plant. First came a new gymnasium to appease and occupy the students, followed by a classroom building and a dormitory necessary to meet the demands of the rising enrollment. The new library had to wait, but its character and placement speak to its importance in McCosh's overall scheme.