[3] Upon Passing out from Sandhurst, Cassels was commissioned as a second lieutenant into the Seaforth Highlanders, a line infantry regiment of the British Army, on 30 August 1926.

[4] He was posted to Central India with the 2nd Battalion of his regiment in 1928 and, having been promoted to lieutenant on 20 August 1929, was appointed aide-de-camp (ADC) to his father in May 1930.

[5] A right-handed batsman and right-arm fast-medium/Off spin bowler,[6] he played first-class cricket between 1928 and 1935[7] and also represented the Egyptian national team.

[11] The following year he played for the Egyptian national side against HM Martineau's XI in Alexandria, taking five wickets in the second innings of the visitors.

The division was sent to France in June 1940 to bolster the French Army in the aftermath of the British Expeditionary Force's (BEF) evacuation from Dunkirk.

[13] The division had been assigned a leading role in the invasion of Normandy, which was to take place in spring the following year, and training was extremely tough, conducted in Scotland.

[13] The division had, under Major General Douglas Wimberley, fought with distinction in North Africa and Sicily from 1942–43 as part of the British Eighth Army under General Sir Bernard Montgomery, and, by now a veteran formation, had been brought back to the United Kingdom by Montgomery, upon his promotion to command the 21st Army Group in December 1943, to spearhead the Normandy invasion.

In France, however, the division, now commanded by Major General Charles Bullen-Smith and war-weary, had performed, Montgomery felt, very poorly.

[13] In mid-July, shortly after Cassels took over command of the 152nd Brigade, Montgomery wrote to Field Marshal Sir Alan Brooke, the Chief of the Imperial General Staff (CIGS), the professional head of the British Army, and informed him that, in his opinion (but shared also by Lieutenant Generals John Crocker and Miles Dempsey, GOCs of I Corps and British Second Army respectively), the 51st Division was, under Bullen-Smith, not battleworthy and decided to replace him with Major General Thomas Rennie, who had served with the division in Africa and Sicily.

[13] Cassels's brigade fought briefly in Operation Goodwood, the British attempt to break out of the Normandy beachhead, following up behind the 11th Armoured Division and securing the ground gained, in which positions it remained for the next month.

Cassels was slightly injured, and two nearby officers were both killed, when his brigade HQ was strafed by a friendly US Army Air Force (USAAF) bomber.

The brigade later led the division's crossing of the river Seine on 1 September and, by the evening, the 5th Battalion, Queen's Own Cameron Highlanders had liberated Saint-Valery-en-Caux.

Finally, the brigade carried out an assault crossing of the Noorder canal and, apart from holding the line, was not involved in any further major actions for the rest of the year.

The brigade then advanced across Germany, and was at Bremen by late April, with the end of World War II in Europe following shortly after.

[16] Aged just 38, he was the second youngest British divisional commander of the Second World War, with only Major General Richard Hull, GOC of the 5th Infantry Division, being two months younger, and Cassels himself being only three months younger than Major General Philip Roberts, GOC of the 11th Armoured Division.

"[22] He relinquished command of the division to Major General Eric Bols in early 1947 to return to England, where he attended the prestigious Imperial Defence College.

[23] Promoted to the substantive rank of colonel on 19 August 1947,[24] he became Director of Land / Air Warfare at the War Office in January 1948.

[30] He was made GOC of the 1 (British) Corps on 4 January 1953[31] and, having been promoted to the rank of lieutenant-general on 2 February 1954,[32] became Director of Military Training at the War Office on 15 November 1954.