Sag Harbor's beautiful John Jermain Memorial Library was built in 1910 by Mrs. Russell Sage in memory of her grandfather, Major John Jermain, and presented as a gift to the people of the Village of Sag Harbor. John Jermain served in the Westchester Militia during the American Revolution. Following the War he moved to Sag Harbor and married Margaret Pierson of Bridgehampton. Their daughter, Margaret Pierson Jermain, was the mother of Margaret Olivia Slocum who married Russell Sage, a financier. The desirable property on which the Library is built was bought at a cost of $10,000, and is directly across from Mrs. Sage's then summer home on Main Street. At that time, it was the highest price ever paid for a piece of real estate in Sag Harbor. Augustus N. Allen designed the building, in 1910, in the Classical Revival style. C.A. Folke of New York was named the superintendent of the building operations when work began.
A pediment and four impressive stone Doric columns adorn the front of the fifty-foot by fifty-foot brick structure, constructed at a cost of $70,000. The trim and base of the building are done in stone. The stone lintels of the windows are designed with the Greek key pattern. Other architectural details include wreaths, torches and egg and dart molding. These types of architectural details can be found in other buildings in Sag Harbor, but are usually worked in plaster or wood. In the interior, fluted stone columns, and lintels ornamented with medallions form the octagonal third floor reading room. Bronze torches light the entryways and vestibules. A winding marble staircase leads up to this room that is crowned by a spacious brick, copper and stained glass dome rising sixty feet above the ground. The dome, constructed by the R. Guastavino Company, is one of the more than 1000 they built, including those at the library of the University of Virginia in Charlottesville, the Cathedral of St. John the Divine in New York City, and the National Museum in Washington, DC.