On 1 September 1900 near Warm Bad, Transvaal, South Africa, Lieutenant Wylly was part of a force under Herbert Plumer which engaged a small group of Boers at Rooikop. The Imperial forces captured 100 rifles, 40,000 rounds of ammunition, 7 Boers, 350 cattle, and 2 supply wagons. After the engagement, Wylly was reported to have been severely wounded, along with another Tasmanian officer, and 3 men from the Bushmen.
On 18 September 1900, the London Gazette carried an announcement that Wylly had been granted a commission as a second lieutenant in the Royal Berkshire Regiment, on the nomination of the Governor of Tasmania, backdated to 19 May 1900. On 16 November, this appointment was cancelled for some reason. On 23 November his VC was gazetted, with the following citation:
Tasmanian Imperial Bushmen, Lieutenant Guy G. E. Wylly On the 1st September, 1900, near Warm Bad, Lieutenant Wylly was with the advanced scouts of a foraging party. They were passing through a narrow gorge, very rocky and thickly wooded, when the enemy in force suddenly opened fire at short range from hidden cover, wounding six out of the party of eight, including Lieutenant Wylly. That Officer, seeing that one of his men was badly wounded in the leg, and that his horse was shot, went back to the man's assistance, made him take his (Lieutenant Wylly's) horse, and opened fire from behind a rock to cover the retreat of the others, at the imminent risk of being cut off himself. Colonel T. E. Hickman, D.S.O., considers that the gallant conduct of Lieutenant Wylly saved Corporal Brown from being killed or captured, and that his subsequent action in firing to cover the retreat was "instrumental in saving others of his men from death or capture."