After a short time in a Civil Service office he joined the British South Africa Police (the police force in Southern Rhodesia) on 23 April 1939. From 1943 to 1946 he served with the British Army in North Africa, including Cyrenaica, and reached the rank of Major. He returned to Rhodesia as a police sergeant, and resumed his career.
On 30 June 1965 he retired from the police force as an Assistant Commissioner to take up the position of Private Secretary to the Governor, Sir Humphrey Gibbs.
After the beleaguered Governor eventually resigned and left Government House in 1969 Pestell returned with him (The Governor did not return to the United Kingdom He simply visited to take leave of Her Majesty the Queen Sir John stayed on while Sir Humphrey returned to Southern Rhodesia)to the United Kingdom. His diary of the post UDI years with Gibbs is especially illuminating for the historians of the period, though it is as yet unpublished.
Pestell subsequently became chief adjudicator on immigration appeals at Heathrow Airport, retiring after 17 years. He died in 2005, and was buried in Charlton Hawthorne, Somerset.