Dinwoodie was commissioned a second lieutenant in the 3rd Battalion of the Dorset Regiment on 8 June 1915, and following training was confirmed in his rank on 14 December. He was seconded to the 1/76th Trench Mortar Battery on 6 April 1916. During the First World War he won the Military Cross in May 1916 while attached to the 1/76th Trench Mortar Battery: Dinwoodie was promoted to the temporary rank of lieutenant on 9 June 1916, and to the acting rank of captain on 15 October. He was appointed an assistant instructor at a trench mortar school on 7 November, with the acting rank of captain, and relinquished his temporary lieutenancy on 8 November. He was promoted to the permanent rank of lieutenant on 1 July 1917. He became an observer in the RAF on 19 September 1918, joining BEF/20 squadron on 26 September with the rank of second lieutenant. On 6 October he and his pilot were flying Bristol Fighter, serial A2402, when they were shot down; Dinwoodie was badly wounded. He continued in service as a trench mortar instructor, and was transferred to the unemployed list and relinquished his acting rank of captain on 11 June 1919. Dinwoodie was demobilised on 1 April 1920, and left the service retaining the rank of captain.
On 20 August 1946, German high explosive bombs were being loaded onto barges for disposal at sea when a bomb was dropped and exploded, killing six and injuring 12. Two train loads of bombs were ready for loading and the port area, in the centre of Lübeck, was evacuated. Squadron Leader Dinwoodie and Corporal Roland Garred were sent to the scene and defused one of the remaining bombs in the batch to discover that a faulty fuse had caused the accident. They rendered the remaining 11 bombs safe when another explosion would have not only killed them but set off the bomb laden trains and devastated the town centre. Leading Aircraftman John Hatton, the driver in the bomb disposal team, also assisted in moving the bombs. Awarded the George Cross on 4 February 1947, the citation in the London Gazette praised Dinwoodie for displaying Garred was awarded the George Medal, while Hatton received the British Empire Medal. Dinwoodie relinquished his commission in the RAFVR with effect from 10 February 1954, retaining the rank of wing commander. Hubert's medals and some photographs are held by the RAF Museum at Hendon, London.
