Together with Sir Alfred Ewing he was responsible for the establishment of the Royal Navy's codebreaking operation, Room 40, which decoded the Zimmermann telegram, a major factor in the entry of the United States into World War I. Reginald Hall was born in Salisbury, Wiltshire, eldest son of Captain William Henry Hall, the first head of Naval Intelligence, who married the daughter of the Reverend George Armfield from Armley, Leeds.

The ship visited foreign ports, particularly in Germany which was now seen as the navy's greatest potential enemy, and Hall started the tour with a long list of places to investigate.

Hall and a couple of officers dressed down as sailors and took the yacht on a full speed circuit of the harbour, pretending to break down by the naval dockyard.

The two were captured and served two-and-a-half years of a four-year sentence, before being pardoned in May 1913 as part of a visit by King George V to Germany.

His reputation for unorthodox treatment of his men continued to grow, making it his business to reform recalcitrant sailors rather than simply punishing them.

Ships spent long periods at sea, before the strategic situation became clearer and the fear of immediate German raids diminished.

[10] Hall's health had deteriorated under the stress of continuous sea service, and after three months matters had reached the point where he was obliged to request posting away from the ship.

His seagoing career cut short by ill-health, Hall was appointed Director of the Intelligence Division (DID) by the Admiralty in October 1914, replacing Captain Henry Oliver.

He also encouraged cooperation with other British intelligence organizations, such as MI5 (under Vernon Kell), MI6 (under Mansfield Smith-Cumming) and the Special Branch of Scotland Yard (under Basil Thomson).

That morning the man who had organised the arms shipment, Sir Roger Casement, was arrested in Tralee Bay after disembarking from a German U-Boat.

[12] When the United States broke off diplomatic relations with Germany, intercepted traffic between the US German legation and Berlin dried up as a key source of British intelligence.

The mass arrests of known Sinn Féin activists, following the discovery of a spurious 'German Plot' in Ireland has been interpreted as: a striking illustration of the apparent manipulation of intelligence in order to prod the Irish authorities into more forceful action...when the British Government was unable to provide convincing evidence of a 'German Plot', nationalist Ireland concluded that it had been invented as retribution for the defeat of conscription.'

[13] Room 40's decryptions also led to the capture of Captain Franz von Rintelen, a veteran field agent in the intelligence wing of the German Imperial Navy, who had operated covertly in the still neutral United States and, among many other things, had financed and encouraged strikes by anti-war labor unions, attempted a hostile takeover of the Du Pont corporation, and firebombed munitions ships and armaments factories.

Upon retirement Hall served as a Conservative Member of Parliament (MP) for Liverpool West Derby from 1919 to 1923, then for Eastbourne 1925–1929.

Even in the House of Commons he was still said to be involved in the Zinoviev letter affair in 1924, which led to the victory of the Conservatives in the general election of that year.

Too old to return to active service on the outbreak of World War II, Hall nevertheless served in the British Home Guard until his death.

Despite his retirement from military and political life, Hall by the late 1930s had been identified as an important target person by the National Socialist police apparatus: in early 1940 the Reichssicherheitshauptamt in Berlin, the headquarters of the intelligence service of the SS, added his name to the Sonderfahndungsliste G.B., a list of people residing in the UK, whom the Nazi leadership and/or its intelligence service regarded as particularly important or (from their point of view) dangerous and who for that reason were slated to be tracked down and apprehended with heightened priority by special task forces of the SS, that were to follow on the heels of the occupying forces in case of a successful invasion of the British islands by the Wehrmacht.

Ambassador to the Court of St. James's Walter Page as a "clear case of genius", while American attaché Edward Bell described him as "a perfectly marvelous person but the coldest-hearted proposition that ever was – he'd eat a man's heart and hand it back to him."