P/O Johnnie Allen joined the RAF in June 1937.
He went to 8 FTS (Montrose) on 21st August.
On 18th January 1938, he was on a training flight when he disappeared in fog over Forfarshire. Search parties failed to find him. Early the next morning, an RAF search plane spotted wreckage on Glen Dye Moor, Kincardineshire. Beside the wreck was a rescue party, laying down, spelling out the word 'ALIVE'. An ambulance plane picked up John, who was badly injured.

After a long stay in hospital he finished his training and joined 54 Squadron at Hornchurch on 5th December 1938.
John made 54 Squadron's first claim of the war when he shot down a Ju88 in the Dunkirk area on 21st of May 1940.
On the 23rd of May 1940, he took part in a daring rescue operation. He and Alan Deere escorted their Flight Commander, James Leathart, to France where he was to land a Miles Master trainer and pick up the C.O. of 74 Squadron who had made a forced landing on the airfield at Calais-Marck.
The pick-up was made, with Allen watching from 8,000 feet and Deere circling at low level. An approaching formation of Bf 109's was spotted by John as the Master taxied out for take-off. As a strafing Bf 109 pulled out of its dive, presenting a perfect target, Deere fired a short burst and the aircraft stalled and then crashed into the sea. Deere, climbing to help Allen, crossed the path of two Bf 109's, one of which turned towards him. Deere also turned, firing at the second one, which rolled over and dived away. Pursuing the first one, he caught up at treetop height and pursued him, firing off his remaining ammunition before the German headed for home. Diving towards the coast Deere called up John and was relieved to hear him answer. In the meantime the Master had taken off and headed for Hornchurch. In the action three Bf 109's had been shot down and three others severely damaged with no losses.

During combat on 25th May 1940, John's engine was hit by a cannon shell and he was forced to bale out over the Channel. He was picked up by a destroyer and he returned to the squadron three days later, dressed in sailor's uniform, carrying a kitbag.

John Allen was awarded the DFC on 11th June 1940, and received it from the King in a ceremony at Hornchurch on 27th June.

On 9th July, John shared in the destruction of a He 59.

On 24th July, John's engine was damaged in combat with Bf 109s over Margate.
His Spitfire (R6812) was shot down on convoy patrol by Bf 109's of III Gruppe of JG 26. He was seen making for Foreness in a controlled descent with a dead engine, which suddenly restarted, causing him to try for Manston. The engine stopped again, and John was trying to turn again for Foreness when his aircraft stalled, spun, and crashed near the Old Charles Inn at Cliftonville (time reported was 12:30hrs).

He was buried with full military honours at Margate Cemetery, Kent. He was 24 years old.