He attended the University of Rochester and served in United States Merchant Marine in the South Atlantic between his freshman and sophomore years.
During World War II, he was on leave from the University of Rochester and spent much of his time in the Radiation Laboratory at MIT developing radar devices for the United States Air Force in the European and Pacific theaters.
He returned to the University of Rochester in 1946, but from 1949 to 1951 he was loaned to the Atomic Energy Commission as chief of the Physics Branch, Research Division.
When Platt was recruited to lead the newly founded Harvey Mudd College in 1956, he was a highly respected teacher and physicist at the University of Rochester, where he worked on the design and construction of the 240-million-volt synchrocyclotron.
After serving as eighth president of Claremont Graduate University from 1976 to 1981, Platt returned to Harvey Mudd and continued to teach there well into his 90s.