He almost single-handedly measured every notable tree in the British Isles, founding the Tree Register of the British Isles (T.R.O.B.I.), which held records of over 100,000 individual notable trees at the time of his death.
During the Second World War, he served with the Fleet Air Arm in the Far East. Returning by troop ship in the Red Sea at the end of the war, he pondered his future and decided it would be trees. In 1976, the Royal Forestry Society of England, Wales, and Northern Ireland awarded him its Medal for Distinguished Service to Forestry (Gold Medal) during a Society meeting at Westonbirt. (From a tribute by Esmond Harris, Quarterly Journal of Forestry, January 1996, page 67).
His 1987 book The Guide to Trees of Canada and North America is dedicated to his sister Christine. The book makes occasional oblique reference to a trip to North America in 1976.
Mitchell's Rule states: "If there are tree stumps or felled trunks nearby, count the annual growth rings and measure the trunk circumference to find local growth rates".