Haddock was born to Herbert James Haddock (born 1825) in Rugby, Warwickshire on 27 January 1861. Before working for the White Star Line, Haddock was a lieutenant with the Royal Navy aboard HMS Edinburgh. In 1902, Haddock was appointed a Companion of the Order of the Bath. He was later aide de camp to British royalty.
His White Star Line career included commanding a number of liners, including the RMS Britannic, SS Germanic, RMS Cedric, and RMS Oceanic.
Haddock was also nominally the first commander of RMS Titanic. He signed on as her master at Southampton on 25 March 1912, and then travelled to Belfast to oversee the crew that was assembling there for the ship's delivery trip to Southampton. He was relieved by Edward J. Smith at Belfast on 31 March and then returned to Southampton to take command of Smith's previous ship, RMS Olympic. On 3 April he began Olympic's tenth Southampton-New York-Southampton roundtrip, arriving in New York on 10 April, the day Titanic left Southampton. Olympic was given the radio call sign MKC.
At the time of Titanic's sinking Haddock was sailing Olympic easterly from New York to Southampton, approximately 500 nautical miles (930 km; 580 mi) west by south of Titanic's location. Haddock was informed of the disaster by wireless operator Ernest James Moore at 2250 ET on 14 April. After receiving a CQD call from Titanic, Haddock calculated a new course and headed directly to her. He also sent for an engineer to set the ship's engines to full power. When 100 nautical miles (190 km; 120 mi) from Titanic, at approximately 1600 ET on 15 April, Haddock received a message from Captain Rostron of RMS Carpathia, explaining that continuing on course to Titanic would gain nothing, as "All boats accounted for. About 675 souls saved [...] Titanic foundered about 2.20 am." Rostron requested that the message be forwarded to White Star and Cunard. He said that he was returning to harbour in New York, and recommended that other ships do the same. Subsequently, the wireless room aboard the Olympic operated as a clearing room for radio messages.
In the United States Senate inquiry into the sinking of the Titanic, Haddock was interviewed by William Alden Smith on 25 May 1912. Haddock gave his residence as Southampton, and his employment as a "master mariner".
Seven weeks after the Titanic disaster, Haddock almost ran the Olympic aground on rocks near Land's End. The error was attributed to faulty navigation, and Haddock was under strict observation for his next few voyages.
Haddock was in command of Olympic in 1914 during her attempt to assist HMS Audacious after she had collided with a German mine. Olympic was subsequently laid up until being converted to a troopship at the outbreak of World War I. Haddock was redeployed to command a dummy fleet of wooden dreadnoughts and battle cruisers, and was stationed in Belfast. In 1915, Harold Sanderson, head of International Mercantile Marine, tried to reassign Haddock to captain Britannic when she was converted to a hospital ship. However, the Admiralty refused to release Haddock from his assignment in Belfast. From 5 to 14 May 1917, Haddock sailed from Liverpool to New York City aboard the SS Saint Paul, six months before she was taken over for wartime service. The voyage's manifest shows that Haddock's trip was funded by the Admiralty, and states that his final destination was Newport News, Virginia, where he would be received by the British Navy. According to the 1917 ship's manifest, Haddock had not been back to the United States since 1914. It is not thought that Haddock returned to the White Star Line after the war.
Haddock died in Southampton on 4 October 1946 at the age of 85.
Haddock married Mabel Eliza Bouchette, in Rock Ferry, Merseyside on 13 May 1893. Bouchette was born in Liverpool in c. 1872 and lived with her father, Francis, at 91 Townsend Lane, West Derby, Liverpool. Other sources state that Bouchette was from Quebec. Haddock and Bouchette had four children – Geoffrey (10 January 1895 – 17 September 1916), Ruth (1896 – 26 October 1958), and twins Herbert (21 October 1903 – 1988) and Joan (21 October 1903 – 21 November 1920). Geoffrey was a Lieutenant in the Victoria Rifles of Canada. He was killed in action on 27 September 1917 at the age of 21, and was buried at the Canadian National Vimy Memorial. In the 28 May 1935 edition of the London Gazette, Ruth was described as a spinster. UK censuses and civil registers show that the family lived in Rock Ferry until at least 1904. They later moved to Southampton, living in the suburb of Bitterne. Bouchette died on 11 March 1935.