Born into a lower-middle-class family in Dallas, Texas, Ash was a migrant worker during the U.S Great Depression and graduated from the University of Texas with a BA degree writing privileged pupils' essays to gain money and also for his personal development as an author. Around this time the Spanish Civil War broke out, and the largely apolitical Ash, driven by a hatred of bullies and fascism, decided that if the war was still going when he was old enough to fight (aged 21), he would join the Abraham Lincoln Brigade.
Ash enlisted in the Royal Canadian Air Force at Windsor, Ontario, on 22 June 1940. He did his basic training at No.1 Initial Training School from 20 July 1940 and was promoted to leading aircraftman on graduation on 14 October 1940. Having been accepted for pilot training Ash was posted to No.12 Elementary Flying Training School from where he graduated on 30 November 1940. Posted to No. 31 Service Flying Training School, he learned to fly single-engine fighters. On graduation, he was commissioned on 25 March 1941. Ash was assigned to Embarkation Depot on 3 April 1941 for the voyage to England where he completed a period with Operational Training Unit before joining No. 411 Squadron RCAF. He flew Spitfires in many defensive and offensive missions, including an attack on the German battleships Scharnhorst and Gneisenau. In 1942, he flew in "big wing" fighter sweeps over France with No. 411 Squadron RCAF.
On one of these missions to attack Comines Power Station on 24 March 1942, flying Supermarine Spitfire Mark Vb (serial number "AB281") from RAF Hornchurch Ash was one of three of the squadron's pilots shot down by Jagdgeschwader 26 he crash-landed at Vieille Eglise about 15 miles from Calais and was smuggled by the French Resistance to Lille and onward to Paris. He was arrested in Paris at the end of May 1942 and imprisoned at Oflag XXI-B, Szubin. In September 1942 he exchanged identities with an army private and joined a fatigue party from which he escaped only to be recaptured the same night. In the spring of 1943, Flight Lieutenant Ash and 32 others escaped from Oflag XXI-B through the latrine tunnel with Harry Day and Peter Stevens. With a companion he tried to reach Warsaw, but was recaptured four days later. Shortly afterwards he was transferred to Stalag Luft III, Sagan, where he was an active member of the escape committee. For the next 21 months, when other ranks were being transferred from Sagan to Stalag Luft VI, Heydekrug, Ash changed his identity and accompanied them. Under his direction a tunnel was later made for a mass escape, but the tunnel was discovered when 10 prisoners had got away. Ash continued the attempt and eventually gained his freedom. He boarded a goods train for Kovno, but was discovered by station guards and returned to Sagan.
His de-briefing after liberation from captivity in April 1945 records the places where he was imprisoned as Dulag Luft (Oberusel) for about three days in June 1942, then Stalag Luft III at Sagan from June to September 1942 Oflag XXI-B (Schubin), where he was held from September 1942 to April 1943, back to Stalag Luft III for April and May 1943, and then Stalag Luft VI (Heydekrug) from May to August 1943 before return to Stalag Luft III for the period August 1943 to January 1945 and finally the naval camp Marlag Milag Nord at Westertimke from January to April 1945.
Ash was reportedly twice sentenced to death as a spy. On one of these occasions the Luftwaffe successfully argued that they should have custody of Ash because he was an airman, thereby taking him from the Gestapo who had sentenced him to death.
On 17 May 1946, he was appointed MBE for his escaping activities. He ended the war as a flight lieutenant.
In August 2015, the BBC reported: "When Ash died aged 96 last year his obituaries noted that he was said to have been the model for Virgil Hilts, the lean, leather-jacketed airman played by Steve McQueen in the 1963 film The Great Escape". The character Hilts, nicknamed "The Cooler King" because of the time he spends in the prison camp's punishment block (which the prisoners call the "cooler") for his persistent escape attempts, steals a motorbike and tries to escape to Switzerland but is caught while using it to jump barbed-wire barricades. The BBC report noted that "Ash modestly denied the claim. For one thing he didn't ride a motorbike, he said. For another, he did not take part in the breakout from the Stalag Luft III camp, on which the movie is based. But the reason he did not participate is that he was locked up in the 'cooler' […] as punishment for another escape attempt".
Demobilised back in England at war's end, he discovered that the act of "taking the King's shilling" in 1939 had robbed him of his US citizenship and that he was now a stateless person. Ash was appointed MBE, awarded British citizenship and went up to Balliol College, Oxford, on a veteran's scholarship, to read PPE. He then joined the BBC, working alongside a young Tony Benn, who became a lifelong friend. Sent to India as the Corporation's main representative on the subcontinent, he was influenced by Nehru's brand of socialism, and by the time he returned to Britain in the late 1950s his politics had solidified into a hard-boiled Marxism. He became involved in left-wing "street politics", including the post-war anti-fascist movement, but his late-blooming revolutionary tendencies eventually proved too much for the BBC, which fired him – though he managed to cling on to freelance employment in the radio drama department as a script reader . Beginning in the 1960s, Ash wrote a series of novels, including Choice of Arms and Ride a Paper Tiger. Politics, however, remained his chief interest. Finding him too quirky and individualistic, the Communist Party rejected his application for membership, and he co-founded the Communist Party of Britain. He also brought his academic background to bear on the subject, publishing a study entitled Marxist Morality. In later life Ash served for several years as chairman of the Writers' Guild of Great Britain and helped to encourage young writers through his work as a script reader for BBC Radio and later as literary manager at the Soho Theatre. His book The Way to Write Radio Drama remained the best on the subject for more than 20 years. Later, he was able to work as a freelance for the BBC's radio drama department as a script reader.
Ash's first marriage, to Patricia Rambault – with whom he had a son and a daughter – was dissolved. In 1955 he married his second wife, Ranjana Sidhanta (1924–2015).