First became interested in aviation whilst working at the Empress Motor Works, Manchester a year later he went to Brooklands to be a mechanic for the French pilot, Maurice Ducrocq. In 1914 joined the Royal Naval Air Service as an instructor, having held a flying certificate for two years. In 1916 he was posted to the Middle East, where he was a member of the Number 2 wing in the Eastern Mediterranean. Operating out of the base in Mudros, Alcock made many daring bombing raids.

He nearly lost his life in 1917 when his plane experienced engine failure and he and his crew were forced to ditch their plane at sea. When they swam ashore, they were taken prisoners by the Turks. After the armistice, he was released, and he left the Royal Air Force in March of 1919.

On June 14, 1919, Alcock, along with his navigator, Lieutenant Arthur Whitten Brown, left St. John's, Newfoundland and flew across the Atlantic, a distance of 1,960 miles to Clifden, Ireland in 15 hours and 57 minutes. For this record-breaking flight he was created Knight Commander Order of the British Empire, by George V at Buckingham Palace on June 21, 1919.

On December 18, 1919, Sir John Alcock, KBE, DSC, flew to Paris to exhibit a Vickers Viking amphibian aircraft. On his landing at Cote d'Evrard he sustained a skull fracture. He was taken to a hospital in Rouen, but he never regained consciousness and he died that very same day.