The benches were more appropriate in the day of horse drawn vehicles. The colonnade was designed by Carrere & Hastings in 1909.

In 1886, he started his practice in partnership with John M. Carrère (1858-1911). They were immediately successful and teamed up with the financier Henry M. Flagler (1830-1913), who was John D. Rockefeller's partner in Standard Oil Company. Flagler wanted to promote Florida as a tourist destination. In 1886, Flagler bought the Jacksonville, St. Augustine & Halifax River Railroad and extended its line to the Florida keys. All along the railroad route he developed hotels and resorts. Flagler sent Carrère and Hastings to Spain to study Spanish and Moorish architecture, with the idea of creating a kind of Riviera in the city of St. Augustine, FL. In 1887-1889, they designed three major buildings in St. Augustine in Spanish Renaissance Revival style: the Grace United Methodist Church (1887), the Hotel Ponce de Leon (1887-88), and the Alcazar Hotel (1887-89). All three buildings incorporated Moorish Revival elements.

Carrère and Hastings are best known for the New York Public Library Main building (1897) at 42nd Street and 5th Avenue. They also designed the Customs House near Battery Park (1899); the Post Office building (1908) at 34th Street and 8th Avenue, and the Municipal Building (1908).

Works in the Washington, DC area include: the Corcoran Gallery of Art (1892-93), the Baltimore Courthouse (1894), the Memorial Bridge (1900), the campus design for Johns Hopkins University (1904), the Pan American Union Building (1907).

After Carrère's death in 1911, Hastings designed the Federal Reserve Building in Washington, DC, the memorial amphitheater in the National Cemetery at Arlington, VA,and other public buildings around the country: the State Capitol and the Museum of Fine Arts in Missouri, and the Hamilton County Courthouse in Cincinnati, OH.

Hastings was critical of the skyscraper, describing it as 'bad for city traffic and the health of the citizenry.' He admired the example of European cities and the growing American Capitol at Washington, DC — cities with building heights limited to about eight stories.

Back in the United States, he worked on panoramas for NYC and Chicago. Then he went to work for McKim Meade & White as a draftsman where Hastings was also working. They left to start their own firm in 1885. They first designed the Ponce de Leon and the Alcazar Hotels in St. Augustine, FL, for Henry Flagler. They followed with a Methodist and an Episcopalian church for St. Augustine. Back in New York, they produced a string of buildings, usually in the Renaissance style. They famously designed the Public Library at 42nd Street and Fifth Avenue, and the Staten Island Borough Hall. The firm went on to design 15 Carnegie Libraries; the City Hall in Portland, Oregon; and Woolsey Hall at Yale. The St. John's Park and Hamilton Fish Park in New York were designed by them. They designed the architectural decorations for the Manhattan Bridge.

Homes or estates for New York elite including: Elihu Root, E. H. Harriman, Mary Guggenheim, Otto Kahn and H.M. Flagler were designed by the partners. Carrere was on commissions that helped plan parts of Cleveland, Ohio; Baltimore, MD; and Grand Rapids, MI. He was an academician in the National Academy of Design and a director of the National Academy in Rome and a member of the Century Club. His end was tragic. He was in a taxicab that was struck by another vehicle and he suffered a concussion. Carrere never fully regained consciousness, and died surrounded by his family.