Porteous was commissioned in the Royal Regiment of Artillery in 1937. On 26 August 1940 he was promoted to lieutenant. On 19 August 1942, he was 24 years old and a temporary major attached to No. 4 Commando when he did the deed for which he was awarded the VC, during the Dieppe Raid. The citation was published in a supplement to the "London Gazette" of 2 October 1942 and read: