In 1904, Aitken carved a 15-foot (4.6 m) statue of a female figure, representing the Republic for the William McKinley Memorial, which still stands in the San Francisco Panhandle Park.

[4] His works include the Science fountain and Great Rivers statues at the Missouri State Capitol, the Iron Mike statue at Parris Island, South Carolina, several military sculptures at West Point and the Temple of Music, and sculptural works for the Liberty Memorial in Kansas City, Missouri.

Perhaps his most famous work is the West Pediment of the United States Supreme Court building, which bears the inscription "Equal Justice Under Law".

[5] The sculpture, above the entrance to the Supreme Court Building, is of nine figures—the goddess of Liberty surrounded by figures representing Order, Authority, Council, and Research.

Aitken himself is depicted in the pediment, seated to the proper left of Liberty with Chief Justice Charles Evans Hughes.