Moving on from the classical and baroque Beaux-Arts repertory, they often designed in the neo-Georgian and neo-Federal styles, and many of their buildings were clad in brick with limestone or white marble trim, a combination which came to be their trademark.

Furthermore, Delano's father Eugene had made partner with the Philadelphia house of Brown Brothers & Co., meanwhile his mothers middle name is Magoun.

He met his longstanding partner, Chester Holmes Aldrich, when they worked together at the office of Carrère and Hastings in the years before the turn of the 20th century.

Delano & Aldrich tended to adapt conservative Georgian and Federal architectural styles for its townhouses, churches, schools, and a spate of social clubs for the Astors, Vanderbilts, and the Whitneys.

Delano alone won the commission for the second-largest residence in the United States, Oheka, overlooking Cold Spring Harbor on Long Island, New York for financier Otto Kahn.

Built from 1914 to 1919 in French chateau style, with gardens by Olmsted Brothers, Oheka ranges over 109,000 square feet (10,000 m2) and was staffed with 125 people.

In 1922, Delano designed the interiors of the Grand Central Art Galleries, an artists' cooperative established that year by John Singer Sargent, Edmund Greacen, Walter Leighton Clark, and others.

As he wrote in the 1934 catalog: Pursuing our purpose of putting American art prominently before the world, the directors a few years ago appropriated the sum of $25,000 for the erection of an exhibition building in Venice on the grounds of the International Biennial.

[citation needed] In Washington, D.C., Delano was the architect for the 1927 renovation to the White House, which later led to structural problems and rebuilding during the Truman Administration.

Delano served on the board of design for the 1939 New York World's Fair and consulted on the controversial White House Truman Balcony in 1946, prior to the reconstruction project of 1949–52.