His public buildings in the Beaux Arts style reflect the optimistic American sense that the nation was heir to Greek democracy, Roman law and Renaissance humanism.

"[7] Historians Christen and Flanders wrote that his reputation among architectural critics went into eclipse during the age of modernism, but has since rebounded because of "respect for the integrity and classic beauty of his masterworks".

[8] Gilbert was born in Zanesville, Ohio, the middle of three sons, and was named after the statesman Lewis Cass, to whom he was distantly related.

[13] Gilbert worked for a time with the firm of McKim, Mead & White before starting a practice in St. Paul with James Knox Taylor.

Many of his Minnesota buildings are still standing, including more than a dozen private residences (especially those on St. Paul's Summit Avenue), several churches featuring rich textures and colors, resort summer homes, and warehouses.

[3] Modernists embraced his work: artist John Marin painted it several times; even Frank Lloyd Wright praised the lines of the building, though he decried the ornamentation.

Gilbert was one of the first celebrity architects in America, designing skyscrapers in New York City and Cincinnati, campus buildings at Oberlin College and the University of Texas at Austin, state capitols in Minnesota and West Virginia, the support towers of the George Washington Bridge, railroad stations (including the New Haven Union Station, 1920),[18] and the United States Supreme Court building in Washington, D.C. His reputation declined among some professionals during the age of Modernism, but he was on the design committee that guided and eventually approved the modernist design of Manhattan's groundbreaking Rockefeller Center.

Gilbert wrote to a colleague, "I sometimes wish I had never built the Woolworth Building because I fear it may be regarded as my only work and you and I both know that whatever it may be in dimension and in certain lines it is after all only a skyscraper.