The 2nd Battalion sailed on the Avondale on 2nd December 1899, and arrived at the Cape about the 25th. They were sent round to Durban. Along with the 2nd Dorsetshire Regiment and 2nd Somerset Light Infantry they formed the 10th Brigade under Major General Talbot Coke, and part of the Vth Division under Sir C Warren, and later under General Hildyard. The 2nd Warwicks and 1st Yorkshire Regiment sailed as part of the 10th Brigade, but were left in Cape Colony. For work of the brigade see 2nd Dorsets, and of the Natal Army generally 2nd Queen's, Royal West Surrey.
The Middlesex were part of the garrison of Spion Kop on the fatal 24th January (see 2nd Royal Lancaster Regiment), and lost heavily, their casualties being approximately 3 officers and 20 men killed, and 5 officers and 60 men wounded. Two men were recommended for the distinguished conduct medal for very exceptional gallantry. In his despatch of 30th January 1900, General Buller bore testimony to the gallant and admirable behaviour and endurance of the troops who were on the hill, including the Middlesex Regiment, who "magnificently maintained the best traditions of the British army".
In the fighting between 13th and 27th February the battalion was several times engaged, but its losses were not serious. Lieutenant Colonel A W Hill was mentioned in the despatch of 30th March 1900.
At the battle of Alleman's Nek (see 2nd Queen's and 2nd Dorsets) the Middlesex Regiment assisted in taking the hill on the right of the nek and did well. Their losses were approximately 1 killed and 9 wounded.
In his final despatch of 9th November 1900 General Buller mentioned 4 officers and 7 non-commissioned officers and men of the battalion, and 8 officers and 19 non-commissioned officers and men were mentioned in Lord Roberts' final despatch.
During the second phase of the campaign the battalion was mainly employed in the south - east of the Transvaal, and in the Newcastle district. They were represented in the garrison of Fort Itala when that place was attacked on 26th September 1901 (see 2nd Royal Lancaster). The battalion had no opportunity of gaining distinction after Alleman's Nek, but 2 officers and 2 non-commissioned officers were mentioned in despatches near the close of the campaign, and in the supplementary or final despatch 4 officers and 5 non-commissioned officers were mentioned.