Max Theunissen joined the Pretoria Commando as a Burgher on 29 September 1899. He took part in all the Natal operations (Talana, Colenso, Waggon Hill, etc) and was promoted to Field-Comet of the Pretoria Commando after some 3 month;. He was wounded in the arm at Spion Kop (25 January 1900) and also slightly in the ear during the Relief of Ladysmith, but did not apply for a Lint voor Wonden.
In May/June 1900 the remnants of the Pretoria Commando, some 150 strong with Max Theunissen as O/C, were operating between Pretoria and Johannesburg. Deneys Reitz joined them (see “Commando”, p. 115-6), and took part in their unsuccessful attempts to destroy the English lines of communications. During one of the skirmishes in July a British lyddite shell exploded after passing through the bunk of a willow bee behind which a group of Burghers had sheltered. A number of Boers were blown up on the spot. Seven killed according to Reitz in “Commando” (p. 117X and 5 were killed and 4 others wounded according to Yardley in “With the Inniskilling Ikragoons” (p. 125).
During 1901 Theunissen served as a Captain (a rank usually associated with a Scouting Corps) in the Fauresmith Commando, under Genls. Hertzog and De Wet. This was also the rank recorded in the British PoW register when Theunissen was captured at Driebad (near the Free State/Cape border) on 5 December 1901.