During the American Civil War, Miller was a manservant for his owner's brother, a Confederate Army captain, and may have been enrolled as a regular soldier after the Battle of the Wilderness in May 1864.

[4] In September 1861, Robert's brother, Captain John J. McBride of Company E of the Fifth Texas Infantry Regiment in the Confederate Army, secured Miller as a personal servant.

When Captain McBride was wounded in the Second Battle of Bull Run in late August 1862, Miller attended him in the hospital until his recovery.

At the Battle of the Wilderness, the Fifth Texas was a part of a charge wherein Captain McBride had both legs broken and was believed to be mortally wounded on the morning of May 6, 1864.

Captain J. E. Anderson of Company C reported that Miller was with the wagon train and did not hear of McBride's injuries until he arrived at Spotsylvania Court House on May 8.

On the 10th, during the Battle of Spotsylvania Court House, Miller was bringing rations to McBrides' company and was forced to run across an open field under fire from Union sharpshooters.

He died on February 25, 1921, and was buried in Lexington, Virginia in Evergreen Cemetery at the side of his brother Johnson "Samuel" Miller.

The remains of his mother Hannah and step-father, William Kenney were disinterred from Clearbrook in Frederick County Virginia and also buried in Evergreen Cemetery as requested in Levi's will.