The interior of the main library building is richly decorated, with a polychrome rotunda. It leads to three public rooms: Morgan's private study to the west, the librarian's office to the north, and the original library to the east. The rotunda has a ceiling with murals and plasterwork inspired by Raphael, created by H. Siddons Mowbray. This ceiling contains themed murals in the lunette panels, which allude to material in Morgan's collection, as well as a central dome, which contains roundels and rectangular panels with various figures or motifs. The rotunda floors are clad with multicolored marble, the pattern of which is based on the floor of the Villa Pia in Vatican City. The walls contain mosaic baseboards and are separated into panels with vertical pilasters, topped by Composite style pilasters. The doorways to the rooms on the east and west are made of white marble, topped by marble entablatures and flanked by green marble columns.