The Mortimer Building was a 19th-century building located at Wall Street and New Street in the Financial District of Manhattan, New York City. It was built by W.Y. Mortimer beginning on June 1, 1884, and completed for occupancy in March 1885. The architect was George B. Post. It fronted Wall Street for a distance of 57 feet (17 m) and New Street for 65 feet (20 m). Used entirely as an office building, the structure adjoined the New York Stock Exchange Building on the west and south. Tenants included lawyers, brokers, and bankers. The building was used as the general headquarters of the Industrial Workers of the World at its peak between 1912 and 1917.