Miller was commissioned as a second lieutenant in 1983 into the Infantry Branch after graduation from United States Military Academy at West Point.

From September 2011 through August 2012, Miller was special assistant to the Director of the Joint Improvised Explosive Device Defeat Organization in Arlington, Virginia.

[11] In 2018, he assumed command of United States Forces — Afghanistan and NATO's Resolute Support mission, after a successful June 2018 visit to the Senate Armed Services Committee.

[13] On 18 October 2018, Miller was in the room at the governor's compound in southern Kandahar when a Taliban gunman fatally shot provincial police chief Abdul Raziq.

[18] On 1 July 2021, Miller gave an exclusive on-camera interview to ABC, with a helicopter flyover of the Bagram Air Base to emphasize its emptiness.

[19] Miller officially furled the mission flag and marked the symbolic end to Operation Resolute Support on 12 July 2021.

U.S. Forces-Afghanistan has reported almost 900 fake accounts posing as Miller on sites like Twitter, Facebook and Instagram just during the first few months of 2021.

[29] The accounts are largely used to trick people out of money and items like gift cards and cellphones, USFOR-A warned.