At the start of World War I, the Greys mobilised and immediately departed for France, arriving on 17 August 1914.
He fought in the Battle of Mons and the subsequent retreat, during which his horse, White Knight, was killed.
In early 1916 the Greys were serving near Vermelles when Cornwallis was awarded the Military Cross for leading bombing arrangements whilst under heavy fire following the explosion of a mine at the front line.
[1] In the autumn of 1917, Cornwallis was attached to 5th Cavalry Brigade headquarters, with his promotion to captain being confirmed on Christmas Day 1917.
After the Armistice, he served with the General Staff in Belgium and was appointed as aide de camp to Field Marshal Haig before being Mentioned in Dispatches in July 1919.
[1] He succeeded his father as honorary colonel of the Thames and Medway Heavy Brigade, Royal Artillery, holding the appointment from 25 September 1937[3] until its disbandment in 1956, and was then Hon Col of 5th Battalion, The Buffs, between 1957 and 1967 and its successor, 8th Battalion, The Queen's Regiment, from 1967 to 1968.
Cornwallis was serving under Field Marshal Haig when he played in a charity cricket match, taking a hat-trick.
[1] Cornwallis played first-class cricket fairly regularly for Kent between 1919 and 1923 as well as making one appearance for the Army in 1920.
[5] He was capped by Kent in 1923 and captained the side after his retirement from the Army between 1924 and 1926, playing much more regularly during this period.
[5] Cornwallis was 27 when he made his first-class debut for Kent and, although he was "a genuinely fast bowler" his body was unable to withstand the rigours of consistent bowling.
He became a Justice of the Peace in 1926 and succeeded his father as Baron Cornwallis in 1935, his older brother having died in 1921 during the Irish War of Independence.
[7] The organisation's main charity is named after the Cornwallis family and was established following the death of the first Baron in 1935.
He was awarded an honorary doctorate Civil Law by the University of Kent and became a Knight of the Most Venerable Order of the Hospital of St. John of Jerusalem.