He was educated at Arbroath High School, a grammar school in Arbroath, Scotland. He graduated in 1982. In 1983, he studied for a Certificate in Pastoral Studies (CPS).
He served as Chaplain-in-Chief of the RAF and head of its Chaplains Branch from 2006 to 2009. In 1974, Mills joined Aberdeen City Police, which became Grampian Police a year later, as a police constable. He served for 4 years.
In 1978, he began the steps to become a religious minister by starting a Bachelor of Divinity (BD) degree at the University of Aberdeen. He then served for a year as an assistant minister in Montrose, Angus. Military career Mills was commissioned into the Chaplains Branch, Royal Air Force, on 13 May 1984.
He was granted the relative rank of flight lieutenant. RAF chaplains hold only relative ranks as they do not hold executive authority. On 13 May 1985, he was promoted to the relative rank of squadron leader.
On the 2 September 1987, he transferred from a short service commission to a regular commission. In 1989, he undertook a four-month posting to the Falkland Islands as Force Chaplain. He was promoted to the relative rank of wing commander on 13 May 1998.
On 27 March 2006, he was promoted to the relative rank of air vice-marshal, and became the 20th Chaplain-in-Chief of the RAF.
In 2001, he was appointed Principal Chaplain (Church of Scotland and Free Churches), and promoted to the relative rank of group captain on 31 May of that year.
