John was born in the Epsom area to Edwin John Spencer and his wife Julia Constance Ogilvie. He had a sister. His father was an insurance broker whose father had been a dressing case manufacturer. His mother was one of the thirteen children of Colonel Patrick Ogilvie of the Madras Army.
In 1911, the Spencers were living at Findon Place, Sussex which they rented from Colonel E W. Margesson. They had nine indoor staff.
John was educated at Summer Fields and Eton. He Matriculated in 1923. The Spencers were living at Knappe Cross near Exmouth. He was rusticated for a couple of weeks at the end of Trinity term 1924, for being caught climbing in to college out of hours.
After leaving Oxford, he became an Underwriter at Lloyds.
His father died in 1931, leaving £307,968-14-11. In the same year, John married Edith Joan only child of Sir Reginald Clarke, formerly Commissioner of Police, Calcutta at St. George’s Hanover Square.
In October 1937, they went to Jamaica on holiday giving their address as 43 Clarges Street, London W.
He was gazetted as a Second Lieutenant in the Welsh Guards on 6 October 1939 and was granted unpaid Acting rank of Captain with effect from 15 September 1941. Based at Codford near Warminster, he went on a three-day Anti-Tank Mine Course at South Petherton on 16 November. From 7 December he was based at Tidworth on a Tactics course which he completed on 3 January 1942.
At the time of D-Day with the rank of Major, he was commanding 2 Squadron, 2nd Battalion, Welsh Guards.
From 6 September until 12 September, Hechtel was the scene of heavy fighting between the Guards Armoured Division and General Student's 1st Falschirm Armee who were sent to reinforce the front between Geel and Hasselt. During the battle, 127 Germans, 623 British soldiers and 35 civilians lost their lives. John was one of them.
He was killed on 9 September 1944 and is buried in the Leopoldsburg War Cemetery Plot IV. A. 11.
In 1953, his widow married the 1st Lord Renwick.