On October 6, 1864, he enlisted in Company H of the 9th Indiana Volunteers, serving as a conscripted soldier until mustering out on September 28, 1865.

In November 1885, Mellette ran unopposed for the office of governor; and, voters selected Huron as the temporary capitol of Dakota Territory.

On November 2, 1889, President Benjamin Harrison signed the proclamation to make South Dakota the fortieth state.

Mellette built a mansion on the bluffs of the Big Sioux River in Watertown, South Dakota.

The Trail of Governors statue of Mellette, by sculptor John Lopez, was unveiled in 2012 and installed in Pierre, South Dakota in front of the Hughes County Courthouse.Mellette's former home in Watertown is maintained as a museum.