Enjoyed a heralded career during World War I and into the 1930's, being promoted all the way to the rank of Major-General. Was considered one of the top counter-espionage experts in the world, both for his methods of investigation and for his knowledge of the subject.

Had an acrimonious relationship with Winston Churchill, Britain's Prime Minister during World War II. Speculation was that Churchill held a grudge against Kell for not sharing classified information with him in the 1920's, a period where Churchill was not in power.

On October 14, 1939, a German submarine sent a torpedo into the side of the British battleship the Royal Oak. When the ship sank, 834 men died. Later, in 1940, an explosion occurred in the Royal Gunpowder Factory in Waltham Abbey. Both incidents were initially considered to have been the result of espionage and sabotage undetected by Kell's MI5. Although later investigations tended to prove that this was not the case, Churchill jumped at the opportunity to cast a dark shadow on Kell. On May 25, 1940, Churchill fired Vernon Kell, removing him from his position as the head of MI5.

Crushed shocked and devastated by his dismissal, Kell, having dedicated his life to serving his country was heartbroken and retired to a small cottage in Buckinghamshire where he died on March 27, 1942.