After returning from Italy, Platt received house and garden commissions from several neighbors in Cornish, and began to attract commissions beyond Cornish, first for gardens and then for entire country estates. Increasingly, Platt's work was also the subject of national publication. Platt continued to design country houses throughout his career, but he devoted much of his time to important urban and institutional commissions after 1920. Many of these commissions came from the Vincent Astor estate office, which employed Platt from 1906 through 1932, and also from residential clients with institutional interests. Previous patron Charles Lang Freer commissioned Platt to design the Freer Gallery of Art in Washington, D.C. in 1913, the first of Platt’s nine museum commissions. Platt also completed or consulted on several large-scale campus planning projects, most notably for the University of Illinois at Urbana-Champaign and for Phillips Academy in Andover, Massachusetts.
Throughout his life, Platt maintained his house and garden in Cornish, New Hampshire, and an office and residence in Manhattan. With his second wife, Eleanor Hardy Bunker, whom Platt married in 1893, Platt had five children. Among the children were William (1897-1984) and Geoffrey (1905-1985), who followed in their father's footsteps and practiced architecture in New York City; the Department of Drawings & Archives also holds the William & Geoffrey Platt archive. Charles Platt died in Cornish in 1933.