The mansion was constructed during the years following 1883 for the financier and politician Herbert Leon in the Victorian Gothic, Tudor and Dutch Baroque styles, on the site of older buildings of the same name.

During World War II, the estate housed the Government Code and Cypher School (GC&CS), which regularly penetrated the secret communications of the Axis Powers – most importantly the German Enigma and Lorenz ciphers.

The GC&CS team of codebreakers included John Tiltman, Dilwyn Knox, Alan Turing, Harry Golombek, Gordon Welchman, Hugh Alexander, Donald Michie, Bill Tutte and Stuart Milner-Barry.

[1] The team at Bletchley Park devised automatic machinery to help with decryption, culminating in the development of Colossus, the world's first programmable digital electronic computer.

[4] The estate of 581 acres (235 ha) was bought in 1883 by Sir Herbert Samuel Leon, who expanded the then-existing house[5] into what architect Landis Gores called a "maudlin and monstrous pile"[6][7] combining Victorian Gothic, Tudor, and Dutch Baroque styles.

[8] At his Christmas family gatherings there was a fox hunting meet on Boxing Day with glasses of sloe gin from the butler, and the house was always "humming with servants".

[22] Key GC&CS cryptanalysts who moved from London to Bletchley Park included John Tiltman, Dillwyn "Dilly" Knox, Josh Cooper, Oliver Strachey and Nigel de Grey.

[25] In one 1941 recruiting stratagem, The Daily Telegraph was asked to organise a crossword competition, after which promising contestants were discreetly approached about "a particular type of work as a contribution to the war effort".

Later-recruited cryptanalysts included the mathematicians Derek Taunt,[31] Jack Good, Bill Tutte,[32] and Max Newman; historian Harry Hinsley, and chess champions Hugh Alexander and Stuart Milner-Barry.

[39] During a morale-boosting visit on 9 September 1941, Winston Churchill reportedly remarked to Denniston or Menzies: "I told you to leave no stone unturned to get staff, but I had no idea you had taken me so literally.

"[40] Six weeks later, having failed to get sufficient typing and unskilled staff to achieve the productivity that was possible, Turing, Welchman, Alexander and Milner-Barry wrote directly to Churchill.

The work was tedious and demanded intense concentration; staff got one week's leave four times a year, but some "girls" collapsed and required extended rest.

[50] Knox's methods enabled Mavis Lever (who married mathematician and fellow code-breaker Keith Batey) and Margaret Rock to solve a German code, the Abwehr cipher.

[39] Alan Brooke (CIGS) in his secret wartime diary frequently refers to “intercepts”:[55] For a long time, the British Government failed to acknowledge the contributions the personnel at Bletchley Park had made.

Jock Colville, the Assistant Private Secretary to Winston Churchill, recorded in his diary on 31 July 1941, that the newspaper proprietor Lord Camrose had discovered Ultra and that security leaks "increase in number and seriousness".

[60] Without doubt, the most serious of these was that Bletchley Park had been infiltrated by John Cairncross, the notorious Soviet mole and member of the Cambridge Spy Ring, who leaked Ultra material to Moscow.

[68] The only direct enemy damage to the site was done 20–21 November 1940 by three bombs probably intended for Bletchley railway station; Hut 4, shifted two feet off its foundation, was winched back into place as work inside continued.

Subsequently, under Group Captain Eric Jones, Hut 3 expanded to become the heart of Bletchley Park's intelligence effort, with input from decrypts of "Tunny" (Lorenz SZ42) traffic and many other sources.

[74] An important function that allowed the synthesis of raw messages into valuable Military intelligence was the indexing and cross-referencing of information in a number of different filing systems.

With the help of German operator errors, the cryptanalysts in the Testery (named after Ralph Tester, its head) worked out the logical structure of the machine despite not knowing its physical form.

While not changing the events, "Ultra" decrypts featured prominently in the story of Operation SALAM, László Almásy's mission across the desert behind Allied lines in 1942.

[114] Knox was given a new section for work on Enigma variations, which he staffed with women ("Dilly's girls"), who included Margaret Rock, Jean Perrin, Clare Harding, Rachel Ronald, Elisabeth Granger; and Mavis Lever.

[115] Mavis Lever solved the signals revealing the Italian Navy's operational plans before the Battle of Cape Matapan in 1941, leading to a British victory.

This led to increased shipping losses and, from reading the intercepted traffic, the team learnt that between May and September 1941 the stock of fuel for the Luftwaffe in North Africa reduced by 90 per cent.

[124] In early 1942, a six-month crash course in Japanese, for 20 undergraduates from Oxford and Cambridge, was started by the Inter-Services Special Intelligence School in Bedford, in a building across from the main Post Office.

In 1999, Michael Smith wrote that: "Only now are the British codebreakers (like John Tiltman, Hugh Foss, and Eric Nave) beginning to receive the recognition they deserve for breaking Japanese codes and cyphers".

[134] June 2014 saw the completion of an £8 million restoration project by museum design specialist, Event Communications, which was marked by a visit from Catherine, Duchess of Cambridge.

[141] In October 2005, American billionaire Sidney Frank donated £500,000 to Bletchley Park Trust to fund a new Science Centre dedicated to Alan Turing.

[162] In April 2020 Bletchley Park Capital Partners, a private company run by Tim Reynolds, Deputy Chairman of the National Museum of Computing, announced plans to sell off the freehold to part of the site containing former Block G for commercial development.

[163][164] Previously, the construction of a National College of Cyber Security for students aged from 16 to 19 years old had been envisaged on the site, to be housed in Block G after renovation with funds supplied by the Bletchley Park Science and Innovation Centre.