Captain F.J. Bertenshaw became the Commanding Officer of the Frontier Mounted Rifles during the Boer War when Major J.R. Hart had his services dispensed with. The F.M.R., a Cape unit raised in 1886, were mobilised in October 1899 and at some stage refused to cross the border into the Orange Free State. Tylden commented that '"During the Boer War the men refused to cross the frontier and the corps was broke" as stated in the words of a serving officer ... There are many difficulties facing Commanding Officers when war breaks out in a country of divided allegiances and ways of thought' (Armed Forces of South Africa refers).

Bertenshaw took over command and the Frontier Mounted Rifles had many sharp engagements during the Cape Rebellion of 1900. 131 Medals were issued named to the F.M.R.