[1] Johnston previously served in Avon and Somerset Constabulary between 1979 and 2004 as Detective Chief Superintendent head of Crime and counter terrorism.
Prior to joining the police, Johnston served in the British Army with 59 Independent Commando Squadron Royal Engineers.
Between 2004 and 2008 Johnston was the Metropolitan Police's head of homicide and serious crime, in which capacity he led the Damilola Taylor and the Torso in the Thames investigations.
From 2008 to 2016 Johnston was the head of the National Technical Assistance Centre, a business unit of GCHQ with responsibility for maintaining interception of communications capability for all UK agencies.
Johnston also held office as the strategic lead for economic crime investigation and policy at Scotland Yard, working with government, industry and international regulatory bodies to help develop anti-money laundering regulation and compliance.