He attended St. Mark's School, then earned a bachelor's degree from Amherst College and a law degree from Harvard Law School in 1926.[2] His elder brother, Sigourney Thayer (1896−1944), was a theatrical producer, aviator and poet.
During World War II, Thayer was commissioned as a lieutenant commander in the U.S. Navy.[1] He was an intelligence officer in the South Pacific early in the war and then went to Europe, where he took part in the invasions of Normandy and southern France. He returned to the Pacific in time for the invasion of the Philippines.[2]
After the war, he was appointed Assistant to the Commissioner of the New York State Division of Housing by then Governor Dewey, and in 1949, Dewey appointed him to lead the New York State Commission Against Discrimination.[1]
In 1950, he began his formal career in diplomacy, as the Assistant U.S. Ambassador to France. He served in that role under Ambassadors David K. E. Bruce, James Clement Dunn, and C. Douglas Dillon until 1954. On August 17, 1955, Thayer was nominated as the U.S. Minister to Romania. He was commissioned during a recess of the U.S. Senate and recommissioned after confirmation on January 25, 1956, presenting his credentials on November 10, 1955, and serving until December 12, 1957.[3] During his term, Soviet troops from Timișoara entered Hungary through the Romanian city of Oradea.[4] In February 1957, he wrote to then Secretary of State John Foster Dulles stating:[5]
"The Romanian Government is as usual straddling the fence with its customary skill. It is following Soviet instructions and its own inclination to keep its people from becoming too keen on us by attacking us publicly from every angle, and at the same time is keeping the door open for such economic and cultural advantages as it may glean at an appropriate moment."[5]