Following his graduation from Kneller Hall, Duncan went on to command and conduct the bands of the Black Watch, Royal Army Ordnance Corps. In 1974 Captain Duncan Beat was appointed the Director of Music of the Scots Guards Band.
In 1980 Major Beat was responsible for the music at the ceremonies in Scotland to mark the 80th Birthday of HM Queen Elizabeth, the Queen Mother. As a result he was appointed a Member (4th Class) of the Royal Victorian Order; this was later re-designated Lieutenant (LVO). At the end of 1982 he was appointed Chief Instructor at the Royal Military School of Music (Army) Kneller Hall.
Following retirement from the army, Duncan accepted the appointment as Director of Music of the Band of the Metropolitan Police London, which he conducted for the next eight years.