The units of the Kenya Army Infantry are the principal fighting arms of the Kenya Army. The primary mission of the Infantry formations is to fight and win land battles within area of operational responsibilities in the defence of the nation against land – based aggression, while the secondary mission is the provision of aid and support to civil authorities in the maintenance of order. The Kenyan School of Infantry (SOI) is located in Isiolo County.

In the early 1960s 3rd, 5th, and 11th Battalions of the King's African Rifles (KAR) were based at Nanyuki, Gilgil (in the same town as 3rd Regiment Royal Horse Artillery, which was at Alanbrooke Barracks) and Nairobi (Langata) in rotation.

Timothy Parsons writes:

'..Kenyan political elites viewed the army as a potential source of political leverage. No party or ethnic group was willing to let its rivals gain a dominant position in the armed forces. As a result, veteran askaris worried that politically connected soldiers would replace them. Most of the "martial races" that comprised the old colonial forces were not part of the Kenya African National Union [KANU], and many Kikuyu openly referred to the KAR as the "KADU army." In 1959, the Kalenjin, Kamba, Samburu, and Northern Frontier pastoral communities supplied approximately 77 percent of the total strength of the Kenyan KAR battalions.'

During the Mau Mau rebellion, "B" Company of the KAR's 5th Battalion perpetrated the Chuka massacre in June 1953, killing a number of Kenyan civilians. The massacre was ordered by the company's commanding officer, Major Gerald Selby Lewis Griffiths. On 11 March 1954, Griffiths was found guilty on five counts; he was sentenced to five years in prison and was cashiered from the Army.

Three KAR battalions were transferred to Kenya upon independence (at midnight on 12 December 1963). Thus 3 KAR, 5 KAR, and 11 KAR became 3 Kenya Rifles, 5 Kenya Rifles, and 11 Kenya Rifles. Their middle and senior ranks were filled almost entirely by regular British officers.

Army mutinies in Tanganyika and Uganda in January 1964 set the stage for the unrest that took place within the Kenya Rifles. Faced with many of the same problems that confronted Kenyan soldiers, Tanganyika Rifles and Ugandan soldiers won improved pay and the dismissal of expatriate British officers by threatening their newly sovereign politicians with violence. On the evening of 24 January 1964, the failure of the Kenyan Prime Minister to appear on television, where 11th Kenya Rifles junior soldiers had been expecting a televised speech and hoping for a pay rise announcement, caused the men to mutiny. Parsons says it is possible that the speech was only broadcast on the radio in the Nakuru area where Lanet Barracks, home of the battalion, was located. Kenyatta's government held two separate courts-martial for 43 soldiers.

In the aftermath of the mutiny and following courts-martial, the 11th Kenya Rifles was disbanded. A new battalion, 1st Kenya Rifles, was created entirely from 340 Lanet soldiers who had been cleared of participation in the mutiny by the Kenyan Criminal Investigations Division (CID).