While Keegan was 13-year-old, he was suffering from tuberculosis, which subsequently affected his gait. Because of the long effects of tuberculosis, he was unable to join military service, because of his young age, he was unable for services in the Second World War. His illness also disturbed him for his education during his early age.
However, Keegan studied at King's College, Taunton. After that, he joined Wimbledon College, where he studied for two years, which help him to enter in Balliol College, Oxford, in 1953. After his graduation, he worked at the American Embassy in London for three years.
Later Keegan was selected in Military History at the Royal Military Academy Sandhurst as a lecturer. After 26 years of his experience as a junior lecturer, he became senior lecturer in military history during his tenure. At the same time, he also held a visiting professorship at Princeton University.
After leaving the academy, Keegan started working as a Defence Correspondent at Daily Telegraph, where he also worked as Defence Editor until his death.
Keegan was a British by nationality and belongs to white ethnicity. He married his long-term girlfriend, Susanne Ingeborg, after dating her for sometimes. The couple blessed with four children and the husband and wife were living a happy married life along with their children. He died on August 2, 2012, because of natural causes at his home in Kilmington, western England.
Keegan was a popular lecturer and military historian, who was earning a tremendous amount of money from his professional career. He had also served as a lecturer for numerous college and military academy, from which he must have collected a huge amount of money. During the time of his death, his net worth was $15 million.