Born to Annie (née Lawrie) and James Mackintosh, Mackintosh was born in Inverness and raised primarily in Tain, a small town in the Scottish Highlands. His mother was a governess and his father was a naval officer. 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. After spending an additional year at school, Mackintosh applied for entry to Britannia Royal Naval College in 1958 to train as a Royal Navy officer and was accepted. In September 1969, he married Sharron Carter, daughter of a Royal Navy officer, and they had two daughters. Mackintosh and Carter subsequently divorced.

Mackintosh’s first novel, A Slaying in September, was published in 1967. He wrote four other original novels between 1967 and 1970; his later books were either based on his television series, or were novelizations of televised episodes of his television series.

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. It was set on board the fictional HMS Hero (F42), which was portrayed by the real Leander-class frigate HMS Phoebe. Mackintosh was appointed a Member of the Order of the British Empire in the 1976 Birthday Honours on his retirement from the Royal Navy.