His grandfather was Army officer Henry Smyth, who was the second son of John Henry Smyth (1780–1822), of Heath Hall, Wakefield, Yorkshire, a Whig MP for Cambridge University (1812–1822) and Lady Elizabeth Anne FitzRoy, daughter of George FitzRoy, 4th Duke of Grafton.

[7] He was 21 years old, and a lieutenant in the 15th Ludhiana Sikhs, 3rd (Lahore) Division, Indian Army during the First World War, when his actions earned him the Victoria Cross (VC).

Lieutenant Smyth succeeded in taking the bombs to the desired position with the aid of two of his men (the other eight having been killed or wounded), and to effect his purpose he had to swim a stream, being exposed the whole time to howitzer, shrapnel, machine-gun and rifle fire.As well as Smyth's VC, the Indian Distinguished Service Medal was posthumously awarded to the men killed during this incident.

The citation for this award, published in the London Gazette, read:[12] For gallantry and initiative at Khajuri, Tochi Valley, on the 22nd October, 1919, when, having been sent forward from Idak to clear up the situation, his quick appreciation, dispositions and leadership averted a serious disaster and contributed largely towards the saving of a valuable convoy attacked by the enemy.

He showed great gallantry under heavy fire, inspired his command, and brought the convoy safely to Idak.In 1923, while serving in India, Smyth played two first-class cricket matches for the Europeans team.

[13] Returning to England, he attended the Staff College, Camberley from 1923 to 1924, and his fellow students included numerous men who would later achieve high command, including Arthur Percival, Dudley Johnson (a fellow VC recipient), Arthur Wakely, Colville Wemyss, Montagu Stopford, John Halsted, Frederick Pile, Gordon Macready, Roderic Petre, Alastair MacDougall, Edmond Schreiber, Michael Gambier-Parry, Richard Dewing, Leslie Hill, Kenneth Loch, Douglas Pratt, Balfour Hutchison, Robert Pargiter, Robert Stone and Henry Verschoyle-Campbell along with Horace Robertson of the Australian Army and Harry Crerar and Georges Vanier of the Canadian Army.

[21] Having managed to engineer leave from India to the United Kingdom in the summer of 1939, shortly before the outbreak of the Second World War, he called in his debt but was disappointed to be seconded to a United Kingdom-based staff job, as GSO1 of the 2nd London Division, a second-line Territorial Army (TA) formation, then commanded by Major General Harry Willans, which had only recently been formed.

He had not recovered from surgery for an anal fistula, which must have caused him significant, and possibly distracting, discomfort, yet managed to be recommended as fit to stay in command by a medical board presided over by the senior doctor in his own division.

[28] It took 16 years and revision of the official history before his version of the affair versus that of General Hutton, his corps commander, was clarified.

Smyth's book, Milestones, 1979, gives his version in which he relates that he had made representations to General Hutton 10 days previously recommending a withdrawal to the west bank of the Sittang River, thus permitting a strong defence line to be established.

One of his sons, Captain John Lawrence Smyth of the 1st Battalion, Queen's Royal Regiment (West Surrey), was killed on 7 May 1944, during the first attack on Jail Hill at the Battle of Kohima.