Washburn's company was federalized in 1861 as part of the 1st Vermont Infantry; he was commissioned as the regiment's lieutenant colonel and second-in-command.

[1] Active in politics as a Whig, Washburn was one of the secretaries of a Windsor County convention organized to support William Henry Harrison in the 1840 presidential election.

[1] Washburn had served as colonel of a Vermont militia regiment from 1837 until 1841,[10][11] and in 1857 he organized the Woodstock Light Infantry company, which he commanded as a captain.

[1][12] In 1861, Washburn's unit was mustered into federal service for the American Civil War as Company B, 1st Vermont Infantry.

[1][13] Washburn also commanded five companies of the 1st Vermont and five from the 4th Massachusetts infantry at the Battle of Big Bethel on June 10, 1861.

[15] Washburn also took charge of Vermont's response following the October 1864 St. Albans Raid by Confederate soldiers based in Canada.

[18] His home was a target, and when Mrs. Smith appeared in the front doorway carrying an unloaded pistol (the only weapon she could find), the raiders decided to bypass the house.

[18] Mrs. Smith then worked to organize the people of St. Albans to mount a pursuit of the raiders, which unsuccessfully attempted to prevent them from escaping to Canada.

[18] Washburn recognized her heroism by appointing her as a brevet lieutenant colonel on the adjutant general's military staff.

[18] Under Washburn's leadership, the adjutant general's office compiled 300 bound volumes of commander's reports, accounts and other records pertaining to service members from Vermont who took part in the war.

[21] In addition to resuming his law practice, Washburn was an organizer of the Woodstock Railroad Company, of which he was elected president.

[19] In 1869, Washburn was a candidate for governor and won the Republican nomination at the party's state convention in June by defeating Dudley Chase Denison and Julius Converse.

[27] As a result of this family connection, Washburn's surviving children became members of the Society of Mayflower Descendants when it was founded in the late 1800s.