During the Second World War, MacMillan served initially in England, putting in place defensive strategies against a possible invasion by the Germans.

He was appointed Brigadier General Staff IX Corps in December 1941, remaining in this post during the Operation Torch landings in North Africa and through to the fall of Tunis in May 1943.

Upon the death of Major-General Thomas Rennie, he assumed command of the 51st (Highland) Division immediately following the crossing of the Rhine on 23 March 1945.

[4] At the age of ten, he joined St Edmund's School, Canterbury, from where he won a Prize Cadetship to attend a shortened course at the Royal Military College, Sandhurst, in April 1915, several months after the outbreak of the Great War.

[8] In April 1916 he was sent to the Western Front where he joined the 2nd Battalion (the 93rd), a Regular Army unit which was then serving as part of the 98th Brigade of the 33rd Division, in Northeast France, and immediately became involved in fierce trench warfare at Brickstacks.

[20] He was promoted to captain on 28 August 1924,[9] serving periodically as a company commander before entering the Staff College, Camberley from 1928 to 1929, where among his fellow students there in his year included several future high-ranking officers, such as Alexander Galloway, Gerard Bucknall, John Harding, Richard McCreery, Philip Gregson-Ellis, William Holmes, Claude Nicholson, Charles Murison, Alexander Cameron, Gerald Templer, Thomas Wilson, I. S. O. Playfair and Leslie Beavis.

[33][9] On 10 April 1940, seven months after the outbreak of the Second World War, MacMillan was promoted to acting lieutenant-colonel and appointed as GSO1 in HQ 55th (West Lancashire) Motor Division, a first line Territorial Army (TA) formation.

[34][35] The division was a motorised infantry formation composed of only two, rather than three, brigades, and was amongst several responsible for coastal defence and for engaging any possible enemy airborne landings in the event of a German invasion.

[36][35] He trained the brigade very hard over the next few months in numerous large-scale exercises until, in late December 1941, he was chosen to be Brigadier General Staff (BGS) in the HQ of IX Corps District.

[37][35] The corps, commanded by Lieutenant-General John Crocker (nine years younger than Nosworthy) from September 1942, embarked from the Tail of the Bank in February 1943 and set themselves up near Algiers in French North Africa on 24 March as part of the 18th Army Group reserve.

[35] The corps, serving as part of Lieutenant-General Kenneth Anderson's British First Army, fought three major battles (Fondouk, Goubellat and Kournine) during the final stages of the Tunisian campaign against German troops and travelled 470 miles over six weeks before entering Tunis on 7 May, just days before the campaign ended, with almost 250,000 Axis soldiers surrendering.

[39] With IX Corps HQ disbanded, MacMillan was transferred briefly as BGS to the First Army headquarters, whose responsibilities included arranging for the victory parade on 20 May 1943 which involved some 26,000 Allied troops of various nationalities.

[35] The 51st Division was selected by Montgomery to take part in the Allied invasion of Sicily, codenamed Operation Husky, where it came under Lieutenant-General Sir Oliver Leese's XXX Corps.

However, the Normandy bocage countryside was ideal for defence, and, despite strong artillery support, the 44th and 46th Brigades encountered heavy resistance on 26 June, the first day of the operation.

[48] The battalion had created a small bridgehead, allowing elements of the 11th Armoured Division to pass through and seize Hill 112, beyond the Odon river.

The "Scottish Corridor", which the Argylls' bridgehead across the river Odon marked the end, and which was now 2,500 yards wide, forced O'Connor, the corps commander, to send in reinforcements to hold it.

After a brief rest the division, on 23 July, transferred to XXX Corps, under Lieutenant-General Gerard Bucknall, another fellow student at the Staff College in the late 1920s, and saw further very tough action to secure the Bois du Homme as part of Operation Bluecoat at the end of July/beginning of August.

[51] That evening, Lieutenant-General O'Connor, GOC VIII Corps, who greatly admired MacMillan wrote in the following words to his wife: "Babe is slightly wounded.

[57] He was subsequently made a Grand Officer of the Dutch Order of Orange-Nassau for his "exceptional valour, leadership, loyalty and outstanding devotion to duty and great perseverance" during the liberation of the Netherlands.

[62][63] MacMillan was promoted to the acting rank of lieutenant-general on 10 February 1947,[64][9] and, three days later, took up his duties as GOC British Forces in Palestine and Trans-Jordan,[65] replacing Lieutenant-General Sir Evelyn Barker, who he had succeeded as GOC of the 49th Division in November 1944, and who was then being sent home amid allegations of having had an affair and his antisemitic order following the King David Hotel Bombing in July 1946.

[67] One unnamed journalist described this as "perhaps the most unpleasant job that has ever fallen to the lot of a British general" but went on to observe that the newly promoted Lieutenant-General MacMillan is "quiet, efficient, yet capable of divine wrath when the need arises: he is a great leader and is both loved and respected by his subordinates.

"[68] Just five days after his arrival, the House of Commons was informed that the British government had decided to place the question of the future of Palestine before the United Nations.

[72] His period in Palestine was marked by increasingly divergent views between the local administration and the British Cabinet in London on the role of the army.

[73] MacMillan recognised the increasing futility of trying to keep the peace between two parties committed to war rather than to cohabitation, and the need to prioritise arrangements for the safe, orderly and timely evacuation of all troops and other British residents as well as 270,000 tons of military equipment and stores.

MacMillan boarded a naval launch in Haifa that would take him to HMS Phoebe on 30 June 1948, "the last man of the British Forces to leave Palestine".

[83][9] This was a period of rising tension between Spain under Franco and Britain over the sovereignty of Gibraltar, which was not eased by the visit in 1954 of the Queen and the Duke of Edinburgh on the last leg of their tour of the Commonwealth.

[85][9] From 1955 MacMillan lived at Finlaystone,[86] his wife's family home on the southern bank of the River Clyde, near the village of Langbank in Scotland.

His family, consisting of his wife Marian, daughter Judy and four sons, George, John, David and Andrew, had been based here during the Second World War and the Palestine assignment.

Apart from doing much, including a lot of manual work, to maintain and improve the house, its garden and the surrounding estate, he immersed himself in Scottish affairs.

[92] Other voluntary work involved him as chairman of the Scottish Police Dependants' Fund and the City of Glasgow Council of Social Service.