During the American Civil War, Newbury served in the 6th Regiment Kansas Volunteer Cavalry, rising to the rank of first lieutenant. He later served in the Army of the Cumberland, fighting for the Union in the Battle of Chickamauga and the battles of Missionary Ridge and Lookout Mountain. After the war, Newbury was appointed Assistant Provost Marshal of the State of Kansas. He passed the bar in Kansas in 1865 and practiced law there for a time. He served as the elected mayor of Iola, Kansas, but resigned in 1870 after deciding to move to the West Coast, where he settled in Portland, Oregon, in August of that year.