When the First World War broke out, Carton de Wiart was en route to British Somaliland where a low-level war was underway against the followers of Dervish leader Mohammed bin Abdullah, called the "Mad Mullah" by the British. Carton de Wiart had been seconded to the Somaliland Camel Corps.^[24] In an attack upon an enemy fort at Shimber Berris, Carton de Wiart was shot twice in the face, losing his eye and a portion of his ear. He was awarded the Distinguished Service Order (DSO) on 15 May 1915.^[25] Upon hearing of the death of the "Mad Mullah", Carton de Wiart stated that he "felt a sense of real personal loss", for the "Mad Mullah" was "a godsend to officers with an urge to fight and a shaky or non-existent bank balance".^[21]

In February 1915, he embarked on a steamer heading for France. Carton de Wiart took part in the fighting on the Western Front, commanding successively three infantry battalions and a brigade. He was wounded seven more times in the war, losing his left hand in 1915 and pulling off his fingers when a doctor declined to remove them.^[26] He was shot through the skull and ankle at the Battle of the Somme, through the hip at the Battle of Passchendaele, through the leg at Cambrai and through the ear at Arras. He went to the Sir Douglas Shield's Nursing Home to recover from his injuries.^[27]

Carton de Wiart received the Victoria Cross (VC), the highest award for gallantry in combat against the enemy that can be awarded to British Empire forces, in 1916. He was 36 years old and a temporary lieutenant-colonel in the 4th Dragoon Guards (Royal Irish), British Army, attached to the Gloucestershire Regiment, commanding the 8th Battalion, when the following events took place on 2/3 July 1916, in the opening days of the Battle of the Somme, at La Boiselle, France, as recorded in the official citation:

Capt. (temp. Lt.-Col.) Adrian Carton de Wiart, D.S.O., Dn. Gds. For most conspicuous bravery, coolness and determination during severe operations of a prolonged nature. It was owing in a great measure to his dauntless courage and inspiring example that a serious reverse was averted. He displayed the utmost energy and courage in forcing our attack home. After three other battalion Commanders had become casualties, he controlled their commands, and ensured that the ground won was maintained at all costs. He frequently exposed himself in the organisation of positions and of supplies, passing unflinchingly through fire barrage of the most intense nature. His gallantry was inspiring to all.

His Victoria Cross is displayed at the National Army Museum in Chelsea.

Carton de Wiart was promoted to Temporary Major in March 1916, and to the rank of Temporary Lieutenant Colonel on 18 July, was brevetted to Major on 1 January 1917 and was promoted to Temporary Brigadier General on 12 January 1917. He was appointed an Officer of the Order of the Crown of Belgium in April 1917. On 3 June 1917, Carton de Wiart was brevetted to Lieutenant-Colonel.. On 18 July, he was promoted to the substantive rank of Major in the Dragoon Guards. He was awarded the Belgian Croix de Guerre in March 1918, and was appointed a Companion of the Order of St Michael and St George in the King's Birthday Honours List in June.

On 8 November, just three days before the end of the war, Carton de Wiart was given command of a brigade with the rank of Temporary Brigadier General.  A.S. Bullock gives a vivid first-hand description of his arrival: 'Cold shivers went down the back of everyone in the brigade, for he had an unsurpassed record as a fire eater, missing no chance of throwing the men under his command into whatever fighting happened to be going.' Bullock recalls how the battalion looked 'very much the worse for wear' when they paraded for the Brigadier General's inspection. He arrived 'on a lively cob with his cap tilted at a rakish angle, and a shade over the place where one of his eyes had been'. He was also missing two limbs and had eleven wound stripes. Bullock, the first man in line for the inspection, notes that Carton de Wiart, despite having only one eye, ordered him to get his bootlace changed.

^{Post-First World War era and the Polish mission}

At the end of the war Carton de Wiart was sent to Poland as second in command of the British-Poland Military Mission under General Louis Botha. Carton de Wiart was appointed a Companion of the Order of the Bath in the 1919 King's Birthday Honours List. After a brief period, he replaced General Botha in the mission to Poland.

Poland desperately needed support, as it was engaged with Bolshevik Russia in the Polish-Soviet War, the Ukrainians in the Polish-Ukrainian War, the Lithuanians in the Polish-Lithuanian War and the Czechs in the Czech-Polish border conflicts. There he met pianist Ignacy Jan Paderewski, Marshal Józef Piłsudski, the Chief of State and military commander and General Maxime Weygand, head of the French military mission in mid-1920. The Ukrainian nationalists under Simon Petlyura were besieging the city of Lwów (Lvov; Lemberg). One of his tasks soon after Carton de Wiart's arrival was to attempt to make peace between the Poles and the Ukrainians; the peace talks were unsuccessful.

From there, he went on to Paris to report to the British Prime Minister, David Lloyd George and to General Sir Henry Wilson, regarding the Polish situation. Lloyd George was not sympathetic to Poland and, much to Carton de Wiart's annoyance, Britain sent next to no military supplies. Then he went back to Poland and many more front line adventures, this time in the Bolshevik zone, where the situation was grave and Warsaw threatened. During this time he had significant interaction with the nuntius (dean of the Vatican diplomatic corps) Cardinal Achille Ratti, later Pius XI, who wanted Carton de Wiart's advice as to whether to evacuate the diplomatic corps from Warsaw. The diplomats moved to Poznań, but the Italians remained in Warsaw along with Ratti.

From all these affairs, Carton de Wiart developed a sympathy with the Poles and supported their claims to eastern Galicia. This caused disagreement with Lloyd George at their next meeting, but was appreciated by the Poles. Norman Davies reports that he (Carton de Wiart) was "compromised in a gun-running operation from Budapest using stolen wagon-lits".  He became close to the Polish leader, Marshal Piłsudski. After an aircraft crash occasioning a brief period in Lithuanian captivity, he went back to England to report, this time to the Secretary of State for War, Winston Churchill. He passed on to Churchill Piłsudski's prediction that the White Russian offensive under General Anton Denikin directed at Moscow would fail, and it did shortly thereafter. Churchill was more sympathetic to Polish needs than Lloyd George and succeeded, over Lloyd George's objections, in sending some materiel to Poland.

On 27 July 1920, Carton de Wiart was appointed an aide-de-camp to the king, and brevetted to Colonel. He was active in August 1920, when the Red Army was at the gates of Warsaw. While out on his observation train, he was attacked by a group of Red cavalry, and fought them off with his revolver from the footplate of his train, at one point falling on the track and re-boarding quickly.

When the Poles won the war, the British Military Mission was wound up. Carton de Wiart was promoted to Temporary Brigadier General and also appointed to the local rank of Major General on 1 January.  He was promoted to the substantive rank of Colonel on 21 June 1922, with seniority from 27 July 1920 and relinquished his local rank of major general on 1 April 1923, going on half-pay as a colonel at the same time. Carton de Wiart officially retired from the army on 19 December, with the honorary rank of Major General