He attended a school in New York on West 13th Street, later entered the New York Free Academy (now the College of the City of New York), but before he graduated left to begin architectural training in one of the large offices in the city.
Following the end of the Civil War Mr. Keller moved to Hartford, and for five consecutive years was in the employ of the Monumental Stone Company, designing both public and private monuments for which there was great demand at that time. In 1870 he opened an architectural office on High, near Asylum Street, and in his early work planned a number of buildings in Hartford including the County Jail, a High School on Hopkins Street, the Pope Factory now occupied as Pratt & Whitney's Office Building, and the Pratt & Whitney Plant. He also became interested in designing homes for the workers in the factories, and twenty-four houses on Columbia Street were built from his plans, so attractive they were soon occupied by leading families of the city. In addition Mr. Keller was the architect of various public and memorial buildings in the state, and throughout his career accepted commissions to design War and Civic Monuments.
A member of the American Institute of Architects over a period of sixty-five years.
Around 1885 he married Mary Monteith Smith (1860-1946) and they had three children: Hilda Montieth Keller (1888-1978), George Monteith Keller, Sr. (1895-1986), and Walter Smith Keller, Sr. (1898-1981).