Born in Mina, New York, Thayer graduated from Hamilton College in 1862 and then joined the United States Army, serving as a major from 1862 to 1865.
He read law in 1868 and went into private practice in the Montana Territory, then in St. Louis, Missouri.
[1] Thayer was nominated by President Grover Cleveland on February 21, 1887, to a seat on the United States District Court for the Eastern District of Missouri vacated by Judge Samuel Treat.
He was confirmed by the United States Senate on February 26, 1887, and received his commission the same day.
[1] Concurrent with his federal judicial service, Thayer was a professor at Washington University in St. Louis, starting in 1890.