[5] The architects were John Carrère and Thomas Hastings, who had earlier designed the Ponce de Leon Hotel and several other buildings in St. Augustine for Flagler.

[6][7] Mary Lily died four years later, and the home was devised to her niece Louise Clisby Wise Lewis, who sold the property to investors.

They constructed a 300-room, ten-story addition to the west side of the building, obliterating Mr. Flagler's offices and the housekeeper's apartment, and altering the original kitchen and pantry area.

[5] Today, Whitehall is a National Historic Landmark and is open to the public as the Henry Morrison Flagler Museum, featuring guided tours, exhibits, and special programs.

In addition to an annual chamber music series, the Flagler hosts the Whitehall lecture series, which brings “experts and best-selling authors to discuss Gilded Age topics, events, and local history.”[9] Past lecture series include historical talks about the dawn of the Progressive Era, World War I, Gilded Age presidents, engineering feats, and Metaphysical America: Spirituality and Health Movements During the Gilded Age.

The Flagler also holds a special exhibition each year, often showcasing Gilded Age paintings, sculptures, glamour photography, or material culture, such as board games, jewelry, cartoons, Tiffany & Co. silver pieces (including ones displayed at the 1893 Chicago World's Columbian Exposition), and women's fashion.

When it was completed in 1902, Whitehall was hailed by the New York Herald as "more wonderful than any palace in Europe, grander and more magnificent than any other private dwelling in the world."

Three stories tall with several wings, the mansion has fifty-five fully restored rooms furnished with period pieces.