During World War II, Duncan served with the Submarine Surveillance Mine Sweeping Service in the Mediterranean and Aegean Seas. On his return from the war, he became a joiner for the Clan Line and Stephen shipyards, whilst taking piping lessons in his free time. He was the first piper to win the Scottish Pipers' Association Knockout Competition in 1964.
In 1974, he moved on to be a bagpipe instructor full-time at the College of Piping in Otago Street, Glasgow; a position he held until 1978 when, he founded his own piping school in Robertson Street.
In his lifetime he would publish three books of traditional and modern pipe music and three solo albums. He was awarded the Balvenie Medal by Messrs Wm Grant & Son in 1996 for his piping achievements.
- Gilchrist, Jim (30 January 2012). "Music Review : Celtic Connections : Spirit of Scotland Pipe Band & Lothian & Borders Police Pipe Band : ****". The Scotsman. p. 34.
- "Duncan Johnstone". The Herald. Glasgow. 27 November 1999. p. 18.
- Gilchrist, Jim (4 August 2010). "Folk, Jazz, Etc: Son's suite tribute to the piper's piper, the late Duncan Johnstone". The Scotsman. p. 40.