Macmillan was born at 52 Cadogan Place in Chelsea, London, to Maurice Crawford Macmillan (1853–1936), a publisher, and his wife, the former Helen (Nellie) Artie Tarleton Belles (1856–1937), an artist and socialite from Spencer, Indiana. He had two brothers, Daniel, eight years his senior, and Arthur, four years his senior. His paternal grandfather, Daniel MacMillan (1813–1857), who founded Macmillan publishing, was the son of a Scottish crofter from the Isle of Arran. He considered himself a Scot.
Schooling, University and early political views
Macmillan received an intensive early education, closely guided by his American mother. He learned French at home every morning from a succession of nursery maids, and exercised daily at Mr Macpherson's Gymnasium and Dancing Academy, around the corner from the family home. From the age of six or seven he received introductory lessons in classical Latin and Greek at Mr Gladstone's day school, close by in Sloane Square.
Macmillan attended Summer Fields School, Oxford (1903–06). He was Third Scholar at Eton College, but his time there (1906–10) was blighted by recurrent illness, starting with a near-fatal attack of pneumonia in his first half; he missed his final year after being invalided out, and was taught at home by private tutors (1910–11), notably Ronald Knox, who did much to instil his High church Anglicanism. He won an exhibition to Balliol, but was less of a scholar than his older brother Dan.
As a child, teenager and later young man, he was an admirer of the policies and leadership of a succession of Liberal Prime Ministers, starting with Henry Campbell-Bannerman, who came to power near the end of 1905 when Macmillan was only 11 years old, and then H. H. Asquith, whom he later described as having "intellectual sincerity and moral nobility", and particularly of Asquith's successor, David Lloyd George, whom he regarded as a "man of action", likely to accomplish his goals.
He went up to Balliol College, Oxford (1912–14), where he joined many political societies. His political opinions at this stage were an eclectic mix of moderate Conservatism, moderate Liberalism and Fabian Socialism. He read avidly about Disraeli, but was also particularly impressed by a speech by Lloyd George at the Oxford Union Society in 1913, where he had become a member and debater. He was a protégé of the then President Walter Monckton, later a Cabinet colleague, and became Secretary then Junior Treasurer (elected unopposed in March 1914, then an unusual occurrence) of the Union and would, in his biographers' view, "almost certainly" have been President had the war not intervened. He obtained a First in Honours Moderations, informally known as Mods (consisting of Latin and Greek, the first half of the four-year Oxford Literae Humaniores course, informally known as Greats), in Hilary Term 1914. With his final exams over two years away, he enjoyed an idyllic Trinity (summer) term at Oxford, just before the outbreak of the First World War.
Volunteering immediately for active service in the Great War, Macmillan joined the British Army and was commissioned as a temporary second lieutenant in the King's Royal Rifle Corps on 19 November 1914. Promoted to lieutenant on 30 January 1915, he soon transferred to the Grenadier Guards. He fought on the front lines in France, where the casualty rate was known to be high, as was the probability of an "early and violent death". He served with distinction as a captain and was wounded on three occasions. Shot in the right hand and receiving a glancing bullet wound to the head in the Battle of Loos in September 1915, Macmillan was sent to Lennox Gardens in Chelsea for hospital treatment, then joined a reserve battalion at Chelsea Barracks from January to March 1916, until his hand had healed. He then returned to the front lines in France. Leading an advance platoon in the Battle of Flers–Courcelette (part of the Battle of the Somme) in September 1916, he was severely wounded, and lay for ten hours in a slit trench, sometimes feigning death when Germans passed, and reading the classical playwright Aeschylus in the original Greek. The then-Prime Minister Asquith's own son, Raymond Asquith, was a brother officer in Macmillan's regiment, and was killed that month.
Macmillan spent the final two years of the war in hospital undergoing a long series of operations. He was still on crutches on Armistice Day, 11 November 1918. His hip wound took four years to heal completely, and he was left with a slight shuffle to his walk and a limp grip in his right hand from his previous wound, which affected his handwriting.
Macmillan saw himself as both a "gownsman" and a "swordsman" and would later display open contempt for other politicians (e.g. Rab Butler, Hugh Gaitskell, Harold Wilson) who, often through no fault of their own, had not seen military service in either World War.
Of the 28 students who started at Balliol with Macmillan, only he and one other survived the war. As a result, he refused to return to Oxford to complete his degree, saying the university would never be the same; in later years he joked that he had been "sent down by the Kaiser".
Owing to the impending contraction of the Army after the war, a regular commission in the Grenadiers was out of the question. However, at the end of 1918 Macmillan joined the Guards Reserve Battalion at Chelsea Barracks for "light duties". On one occasion he had to command reliable troops in a nearby park as a unit of Guardsmen was briefly refusing to reembark for France, although the incident was resolved peacefully. The incident prompted an inquiry from the War Office as to whether the Guards Reserve Battalion "could be relied on".
Macmillan then served in Ottawa, Ontario, Canada, in 1919 as ADC to Victor Cavendish, 9th Duke of Devonshire, then Governor General of Canada, and his future father-in-law. The engagement of Captain Macmillan to the Duke's daughter Lady Dorothy was announced on 7 January 1920. He relinquished his commission on 1 April 1920. As was common for contemporary former officers, he continued to be known as 'Captain Macmillan' until the early 1930s and was listed as such in every General Election between 1923 and 1931. As late as his North African posting of 1942–43 he reminded Churchill that he held the rank of captain in the Guards reserve.
On his return to London in 1920 he joined the family publishing firm Macmillan Publishers as a junior partner, remaining with the company until his appointment to ministerial office in 1940. He resumed with the firm from 1945 to 1951 when the party was in opposition.