Career
He was the father of Field Marshal James Cassels. Educated at Sedbergh School and the Royal Military College, Sandhurst, Cassels was commissioned into the Indian Staff Corps as a second lieutenant on 22 January 1896. He received promotion to lieutenant on 22 April 1898, to captain on 22 January 1905 and to major on 22 January 1914.
He went on to serve in World War I, receiving rapid and successive promotion to temporary lieutenant-colonel on 19 January 1916, brevet lieutenant-colonel on 3 June 1916, brevet colonel in June 1917 and to temporary brigadier-general on 6 August 1917.
In November 1917 he was appointed Commander of 11th Indian Cavalry Brigade and took his brigade on a great turning movement up the left bank of the River Tigris, outflanking the Turks and helping to bring the Mesopotamian Campaign to an education Following the war, he briefly commanded the 18th Indian Division while it was stationed in the Middle East.
Cassels was promoted to major-general on 1 January 1919 in the Cavalry in India when he was appointed Commander of Peshawar District. He became Adjutant-General, India in 1928, and was promoted to lieutenant-general on 14 April (back-dated to 1 May 1927).
He was promoted to general on 15 October 1929 and appointed General Officer Commanding-in-Chief of Northern Command, India in 1930.
He continued in that post into World World War II and retired in 1941.