Platt graduated from Harvard College in 1919, and from Columbia University School of Architecture in 1923. He passed the succeeding decade working in his father, Charles Platt's architectural firm, until establishing an independent practice in partnership with his brother, Geoffrey, in 1934. Some of the more important commissions executed by this partnership are the Corning Building, New York; the Stern House, New Orleans; and a number of buildings for the Deerfield (Massachusetts) Academy, and for Smith College, Northhampton, Massachusetts. This firm also designed the present Academy school building, completed in 1959. Platt was treasurer of the Academy, and thus served on Council, from 1954 to 1961.
Like Faulkner, Platt made his summer residence in Cornish, New Hampshire. The painter described his friend, saying, "he combined the most characteristic qualities of his parents, calm and tempered judgment, rich depth of feeling, and a quick response to the ridiculous."
Platt was on active military duty during World War II. Having been elected an Associate just at the time he entered upon four years' service as a lieutenant commander in the United States Naval Reserve Aviation Corps, he requested and was granted several extensions of the deadline for submitting his qualifying portrait.
