Post served in the American Civil War under General Burnside at the Battle of Fredericksburg and later rose to the rank of colonel in the New York National Guard.

[5] In 1867, Post founded his own architectural firm which expanded in 1904 when two of his sons, J. Otis and William Stone joined him to form become George B.

A true member of the American Renaissance, Post engaged notable artists and artisans to add decorative sculpture and murals to his architectural designs.

In 1905, his two sons were taken into the partnership, and they continued to lead the firm after Post's death, notably as the designers of many Statler Hotels in cities across the United States.

In 2014, curator, architect George Ranalli presented an exhibition of Post's drawings and photographs of the design of the City College of New York's main campus buildings, on loan from the New-York Historical Society.

The Cornelius Vanderbilt II House, which Post designed in partnership with Richard Morris Hunt, was an English Jacobethan Gothic red-brick and limestone chateau that stood at the corner of East 57th Street and Fifth Avenue and was one of the most opulent single-family homes of its time.

It featured a lavishly scrolled cast-iron gate forged in Paris (now in Central Park), sculptural reliefs by Karl Bitter (now in the Sherry-Netherland Hotel), an ornate reddish-brown marble fireplace sculpted by Augustus Saint-Gaudens (now in the Metropolitan Museum of Art), and elaborate interior decoration by Frederick Kaldenberg, John LaFarge, Philip Martiny, Frederick W. MacMonnies, Rene de Quelin, and Augustus Saint-Gaudens and his brother Julius.

Post also designed many of the gilded-age mansions found in Bernardsville, NJ and was credited more than anyone with selling wealthy New Yorkers on the idea of establishing a country home in the Somerset Hills.

In 1893, Post was named to the architectural staff of the World's Columbian Exposition in Chicago, Illinois, by Burnham and Root,[14] where he designed the Manufactures and Liberal Arts Building.

In 1894, Post, along with J. Herbert Ballantine, Robert L. Stevens, and Edward T. H. Talmadge each pledged $8,000 to purchase land in Bernardsville, New Jersey, to establish the Somerset Hills Country Club, which, after being built on the banks of Ravine Lake was relocated in 1917 to its present site[5] and includes a golf course designed by A.W.