He saw active service in the Second World War, taking a prominent role in Operation Ironclad (the Battle of Madagascar) and the Arakan offensive of the Burma campaign, and later advised the British government on ending conscription and reducing the size of the army by fifteen battalions.

[10] In February 1939 he returned to the Staff College, Camberley, this time to serve as an instructor, a post he held until December 1939, three months after the Second World War began.

[8] In September 1940 he became Commanding Officer of the 2nd Battalion East Lancashire Regiment and then in April 1942 he became Commander of 29th Independent Infantry Brigade Group which was the landing force of Force 121 for Operation Ironclad, the seizure of Vichy French ports and airfields in the Indian Ocean, notably Diego Suárez,[11] Majunga and Tamatave[12] in Madagascar.

In November 1942 Festing took command of the 36th Indian Division and at the beginning of 1944 led it in the final stages of the Arakan offensive of the Burma campaign.

[21] After recovering from a blood clot on the brain,[21] he was appointed President of the Regular Commissions Board on 1 October 1950[22] and became Assistant Chief of Staff (Organisation and Training) at Supreme Headquarters Allied Powers in Europe on 1 April 1951.

[23] He took part in the funeral procession on 11 February 1952 following the death of King George VI[24] and was appointed a Knight Commander of the Order of the British Empire in the Queen's Birthday Honours 1952.