Dartnell offered his services to the British Army on the outbreak of the First World War, and was commissioned into the 25th (Frontiersmen) Battalion, Royal Fusiliers in February 1915.

He fought in the East African Campaign and was mentioned in despatches for the Battle of Bukoba, where he had stormed the German-held town hall, pulled down the German flag and replaced it with the Union Jack.

The regiment was mobilised as the 5th Victorian (Mounted Rifles) Contingent in early 1901 for service in the Second Boer War, and embarked for South Africa on 15 February.

[3][5] After he returned to Australia, Dartnell married Elizabeth Edith Smyth on 15 April 1907 at Holt's Matrimonial Agency on Queen Street, Melbourne.

He cabled the War Office with a list of interested names (including his own), offered their services and requested passage to England in order to enlist.

[10] Dartnell had spent the intervening time stationed in Swaythling, and had made a number of trips to Belgium ferrying drafts of artillery horses to the Western Front.

[3][6] The 25th Battalion arrived in Mombasa on 4 May, and was immediately dispatched to Kajiado to defend the local section of the Uganda Railway from German raiding parties.

[1][6] The assault party, numbering approximately 400, had had to sail across Lake Victoria and scale a "cliff-like incline" before they reached the outskirts of Bukoba to commence their attack on 21 June.

In the final assault, Dartnell had been the first to enter the German-held town hall, and had hauled down the German flag to replace it with the Union Jack.

[9] Shortly after the victory at Bukoba, the 25th Battalion moved to Voi where a military railway was being constructed for an allied advance into German East Africa.

His Majesty the KING has been graciously pleased to award the Victoria Cross to the undermentioned Officers: — Temporary Lieutenant Wilbur Dartnell, late 25th (Service) Battalion (Frontiersmen), The Royal Fusiliers (City of London Regiment).

[3] Elizabeth Dartnell received her husband's VC from Sir Ronald Munro Ferguson, the Governor-General of Australia, in a private ceremony at Government House, Melbourne on 7 October 1916.

[18] Elizabeth, who was living with her daughter in the Melbourne suburb of Murrumbeena at the time, later named her house "Maktau" after the place of her husband's actions and death, and was later invited to the VC centenary celebrations in London in 1956.

Several newspaper reports after his death postulated whether his VC was the first awarded to an actor,[20][21][22] and he is commemorated on a plaque in the Theatre Royal, Drury Lane.