Coming with his family to Lockport, Illinois, Arnold worked in his father's store until the outbreak of the Civil War.
Arnold served with the 4th Regiment Illinois Volunteer Cavalry and the Chicago Mercantile Independent Battery Light Artillery, eventually being captured and imprisoned at Camp Ford, Texas.
When the Civil War began the next year, Arnold enlisted in the 4th Regiment Illinois Volunteer Cavalry of the Union Army.
He returned after recovery with the Chicago Mercantile Independent Battery Light Artillery, where he fought at Arkansas Post, Port Gibson, Champion Hill, Big Black River Bridge, Vicksburg, and Mansfield.
He served for four years and then returned to the store in Lockport and engaged in some real estate and loan dealings in Chicago.