Educated at Inverness Royal Academy, Mackintosh initially applied to join the Fleet Air Arm as a pilot but was rejected due to poor eyesight; a subsequent application to join the Royal Air Force was also rejected for the same reason.

Whilst serving as a Royal Navy officer, Mackintosh co-created the popular and acclaimed BBC television drama series Warship (1973–1977) and wrote several of its episodes.

Mackintosh was appointed a Member of the Order of the British Empire in the 1976 Birthday Honours on his retirement from the navy.

Following the end of Warship he switched to Yorkshire Television, creating and writing most of the episodes of Wilde Alliance, Thundercloud, and The Sandbaggers.

On the evening of 7 July 1979, Mackintosh was flying with two others (his girlfriend, Susan Insole, and British Airways captain Graham Barber, who was the pilot) over the Gulf of Alaska in a light aircraft.