He attended the National Academy of Design and the Art Students League[2] in New York, and later, the Académie Julian in Paris, with Gustave Boulanger and Jules Joseph Lefebvre.

A trip to Italy in 1892 in the company of his brother to photograph extant Renaissance gardens and villas led to a marked development in Platt's aesthetic approach.

The volume was strong on the surviving gardens of the Renaissance and Baroque and made no attempt to describe their history or their designers.

The impact of Platt, and of Edith Wharton's Italian Villas and Their Gardens (1904), can be seen in the shift among stylish Americans from country houses set in lawns with shaped beds of annuals, swept drives and clumps of trees typical of 1885 to houses in settings of gravel-lined forecourts, planted terracing, formal stairs and water features, herbaceous borders and pergolas typical of the early 20th century.

His own garden in Cornish, made between 1892 and 1912, exemplifies a new style, essentially an Arts and Crafts setting for Beaux-Arts Neo-Georgian and Colonial Revival architecture.

Platt designed a grand country estate for Edith Rockefeller McCormick at "Villa Turicum" in Lake Forest, Illinois (1912, demolished).

His more visible public commissions include the Italianate palazzo he designed for the Smithsonian Institution's Freer Gallery of Art (1918) in Washington, D.C., and the campuses of the University of Illinois Urbana-Champaign (1922 and 1927), Connecticut College, Deerfield Academy, and Phillips Academy Andover, where he designed the chapel and library and their settings.

These included several buildings (1924–31) combining classrooms and offices, a dormitory, gymnasiums, plus such landmarks as the Main Library, McKinley Hospital, and the President's House.

[4] His Italian Renaissance-styled Russell A. Alger House, at 32 Lakeshore Drive, now serves as the Grosse Pointe War Memorial.