In the Anglo-Boer War (1899-1902) he initially was a corporal in the Boksburg command, but later on he was promoted to general in virtue of his excellent weapon skills.

With General Ben Viljoen Muller led the Boers in the Battle of Helvetia on 29 December 1900, 10 km north of Machadodorp.

They won a surprise victory over the British forces of Major Stapleton Lynch Cotton (1860-1928), attacking them at night from east and west.

[2][5][6] Taking 235 prisoners of war, Muller later became particularly famous for the capture of this cannon, jokingly called the 'Lady Roberts'.

[7] On 12 June 1901 Muller and his troops overpowered about 350 men of the Australian 5th Victorian Mounted Rifles at Wilmansrust, Transvaal seizing ammunition, pom-pom guns, clothing and food, much needed by the Boers at that point.