After attending school and studying law under Cyrus Blanchard, Rawson initially worked as principal of a public school. This ended in June 1862, during the American Civil War, when he enlisted in the 118th New York Infantry Regiment. He served under Brigadier-General Isaac J. Wistar at Yorktown and under Major-General W. F. Smith at Drury's Bluff. He spent a lot of time serving as Chief Clerk of his division or post headquarters, and at one point was in charge of a large corps of clerks in the office of the U.S. Army Hospital in Point Lookout, Maryland. His military service ended in June 1865.