Born in Danville, Illinois, Platt received a B.A. from the University of Illinois in 1914 and a J.D. from the University of Chicago Law School in 1916. He was in private practice in Danville, Illinois from 1916 to 1917. He was in the United States Army during World War I, from 1917 to 1918. After the war, he returned to private practice in Danville until 1933, also working as a city attorney for Danville from 1927 to 1928. He was a judge on the Fifth Circuit Court of Illinois from 1933 to 1949. 