He was the son of Henry Martin Johnston of Belfast Bank, Glenarm.
Educated at Campbell College, Johnston joined Belfast Banking Company on 18th January 1937.
The London Gazette (1944) records a John Laird Johnston with Other Ranks Service Number 1308099 and Officer Service Number 183336 serving with the RAF Volunteer Reserve.
Following the end of WWII, Johnston rejoined the Belfast Banking Company and is recorded as being a member of Portaferry branch.
The Lennon Wylie website records Johnston as attending Campbell College:
2134. *Johnston, John Laird (C), b. 6th March, 1916, son of Henry M. Johnston, Belfast Bank House, Glenarm. V, July, 1932. Bank Official. 1940-45: F/O., R.A.F. Musician, B.B.C., Belfast. Address: 4 Galwally Avenue, Belfast. (M.q.)