At the outbreak of the Second World War, Haining was appointed General Officer Commanding-in-Chief Western Command and moved on to be Vice Chief of the Imperial General Staff in 1940.[3] He was appointed Intendant General for Middle East Forces in 1941:[3] Prime Minister Winston Churchill described the role of an Intendant General to be that of "serving the Commander-in-Chief with the largest possible measure of supplies".[5] He retired from the British Army in 1942.[3]
Following his death, his friend George Richard Hodges Nugent (later Baron Nugent of Guildford) wrote to The Times to remark on Haining's character and post-retirement life:
When he resigned from the Army, despite weighty grounds for complaint, he wasted no time in repining but immediately made himself available for public work of all kinds, from the parish to the county. His talents and personality were not allowed to be wasted for long; he quickly became chairman of his local Bench of Magistrates, a county councillor, and, in 1949 Lord Lieutenant of the county of Surrey, besides a host of lesser appointments. There was no function so great that his presence did not add distinction to it. and no function too small for him to give his time and energy to it. Every day was a working day for him. Saturdays and Sundays, included, and the number of British Legion parades and services, large and small, attended by him must be legion indeed! When he returned home after his last evening engagement he would embark on his large personal and official correspondence, all written in his own strong characteristic handwriting, for secretarial staff was not within his means. His outstanding gift was his broad human sympathy and understanding: every-one great or small who met him felt that they met him on the level and could trust him absolutely. His great energy, shrewd common sense and active brain will be much missed from a wide range of administrative work in the county. Even more, his warm, lovable personality will be missed by a multitude of men and women in every walk of life who knew him as their friend.