The constituency was formed in 1918. Until then Brecknockshire and Radnorshire had been separate seats.
With a total of fewer than 6,000 voters, Radnorshire had one of the smallest electorates of any parliamentary constituency.
The largest settlement in the largest constituency in Wales and England is neither Brecon nor Radnor but the former mining town of Ystradgynlais.
Until two other mining towns - Brynmawr and Cefn Coed y Cymmer - were excluded from the seat by a 1970s boundary revision, Brecon & Radnor was regarded as a safe Labour seat.