 Having completed the Senior Command Course at Bramshill, Johnston moved to Kent Constabulary in 1989, where he served as Assistant Chief Constable in charge of first Administration and Supply, and then Operations, before moving back to the Metropolitan Police in 1992 as a Deputy Assistant Commissioner.In 1994 he was appointed Assistant Commissioner for the South East London area, but in 1999 moved to Assistant Commissioner with responsibility for Territorial Policing in 2000.In September 2009 Sir Ian retired from the British Transport Police and was succeeded by Andrew Trotter OBE QPM

Johnston came to media attention having given evidence before the enquiry and following the publication of the Macpherson Report into the murder of Stephen Lawrence.