Harmon studied for a year at the School of the Art Institute of Chicago. He received his bachelor's degree at the Columbia University School of Architecture in 1901. Following graduation, he worked for nine years in the firm of McKim, Mead and White and was entrusted with the construction of the Metropolitan Museum of Art. From 1913 to 1928 he had an independent practice; after 1928 he became a partner in the firm of Shreve, Lamb and Harmon. His buildings include the Hotel Shelton, New York, the Julliard School (now the Manhattan School of Music), and the YMCA building in Jerusalem. With his partners he worked on the Empire State Building, the New York Times Building, and Olin Hall at Cornell University, Ithaca, New York. He was a consultant for two New York housing projects, Stuyvesant Town and Peter Cooper Village.
Harmon was president of the Architectural League of New York (1933-35) and of the New York chapter of the American Institute of Architects (1937-39).
