On the announcement of the outbreak of the First World War, there was a rush of volunteers to Nairobi, the capital of British East Africa (now Kenya). The volunteers, armed with hunting rifles, organised themselves into a number of irregular units of mounted horse. These were generally named after the sponsoring identity. Some of these units were known as Bowker's Horse, The Plateau South Africans, Wessels Scouts and the Legion of Frontiersmen. Bowker's Horse (led by Russel Bowker) was made up of Boer settlers in British East Africa and The Plateau South Africans were a group of Boer settlers from the Uasin Gishu Plateau. The Legion of Frontiersmen was a unit made up of local members of the British East African Command of the Legion of Frontiersmen.

However the total number of volunteers was too small to justify this number of independent units so they were amalgamated into one regiment, the EAST AFRICAN MOUNTED RIFLES. This regiment of essentially mounted infantry was formed into six squadrons number "A" to "F". Only Bowker's Horse having sufficient numbers to remain a viable subunit of the Regiment, by becoming "B" squadron and retaining their name and the initials B.H. on their helmets. The Plateau South Africans formed the bulk of "D" squadron. At this stage it is not known what squadron the Legion of Frontiersmen went into. It may have been "A" squadron, but more research will be needed to confirm this.

Initially "A" squadron, commanded by Captain A.K. O'Brien, was armed with a Lance in addition to their hunting rifles, the idea seems to have been that a squadron of Lancers would have been useful in slaying large numbers of German Askari. This Squadron was named "Lady Monica's Own", after the Governor of BEA's daughter. Despite much practice the squadron never attained a high level of proficiency with their lances and they were discarded "to the sorrow of the Squadron Commander, but to the relief and safety of the troopers."

The Kenyan (originally the British East African) command of the Legion of Frontiersmen was a large and vigorous command. Active up until its disbandment in 1963 following Kenya's independence from Britain. The last commander was Major Logan Hook.

The E.A.M.R., originally numbering about 400 in total played a vital roll in the defence of British East Africa before the arrival of Commonwealth troops, thereafter its strength gradually diminished as its officers and men were transferred to the newly arriving commonwealth troops and to the expanding corp of the King's African Rifles.

References
Wilson, C.J. (1938) The Story of the East African Mounted Rifles. The East African Standard, Nairobi. (Wilson was a Captain and the Chief medical Officer for the EAMR)