Johnstone was the son of Sir Frederick Johnstone, 7th Baronet and his wife Lady Louisa Craven, daughter of 1st Earl of Craven, and great-grandson of Sir William Pulteney, 5th Baronet. He succeeded to the baronetcy on birth, his father having died previously. He was educated at Eton College and Christ Church, Oxford. He joined the Dorset Yeomanry Cavalry in 1862.
Johnstone was a close friend of the Prince of Wales (later King Edward VII) and was named as a co-respondent in the divorce case involving Sir Charles Mordaunt and his wife, Harriet (1870) in which the Prince was called to give evidence.
At the 1874 general election Johnstone was elected Member of Parliament for Weymouth and Melcombe Regis. He held the seat until 1885. He was a racehorse owner and his horses twice won The Derby - St. Blaise in 1883 and Common in 1891. Owned a property at Westerhall, near Langholm in Dumfriesshire and it is possible that his ownership of St Blaise and Common are the reason that the annual Langholm Common Riding has, as its colours each year, the colours of that year's Derby winner. Johnstone died at the age of 71 in 1913.
- "Salisbury and Winchester Journal, 18 September 1820" in online database  (subscription required) accessed 1 June 2014
- Debretts House of Commons and the Judicial Bench 1881
- Leigh Rayment's Historical List of MPs – Constituencies beginning with "W" (part 3)
- Chest of Books - St Blaise winner of the Derby
- Thoroughbred Heritage - Common