The earliest surviving reference to it is in the Historia Brittonum, which describes a marvelous spring in the regione of Cinlipiuc that has an abundance of fish despite not being fed by a stream. John Edward Lloyd notes that this Cinlipiuc appears to be one of the various district names created by adding the element -wg to a personal name, in this case an unknown Cunalipi or Cynllib. The Domesday Book refers to a place called Calcebuef, which rendered ten shillings; one editor suggests this is a corruption of Cynllibiwg and that it derives ultimately from the name of Saint Cynllo. However, Lloyd connects this name instead to the cantref of Buellt. The Red Book of the Exchequer, a mainly 13th-century compilation, mentions a region of seven cantrefs between the Severn and the Wye that had been known as Kenthlebiac during the time of Rhys ab Owain of Deheubarth (died 1078). This would place Cynllibiwg in the region later known as Rhwng Gwy a Hafren, but reveals nothing about its extent and nature. The editor suggests that seven cantrefs should be amended to three, perhaps Arwystli, Maelienydd, and Elfael; these are mentioned earlier in the list as part of Powys in the time of 'Meic Menbis', but no longer such.
- Historia Brittonum, ch. 70; Nennius: British History, and The Welsh Annals, ed. John Morris (Phillimore, 1980), cap. 70 (p. 81).
- Lloyd, John Edward (1912). A History of Wales from the Earliest Times to the Edwardian Conquest. Longmans, Green, and Co. p. 281. Retrieved May 11, 2010.
- Domesday Book: Herefordshire ed. Thorn and Thorn (Phillimore 1983), A10 (f.179b) and notes on it.
- Darby, H. C. (1986). Domesday England. Cambridge University Press. p. 328. ISBN 0-521-31026-1. Retrieved November 17, 2009.
- Red Book of Exchequer, ed. Hubert Hall (3 vols. HMSO, 1896: Rerum Britannicarum medii aevi scriptores, 99) II, 762.
- Paul Remfry, "Discovering the lost kingdom of Radnor", British Archaeology, no 34, May 1998
- Standard works on Welsh history and early/medieval Wales which do not mention Cynllibiwg at all include the following: John Davies, History of Wales (Penguin, 1992); Sir J. E. Lloyd, A history of Wales from the earliest times to the Edwardian Conquest (revised ed., 1937), the classic survey of the period; R. R. Davies, Conquest, Coexistence and Change: Wales 1063-1415 (Oxford University Press, 1991), probably the most comprehensive and authoritative single volume survey of the period; Wendy Davies, Wales in the Early Middle Ages (University of Leicester Press, 1982), (refers once to Cynllibiwg as a named early region, based on Nennius); J. Beverley Smith, Llywelyn ap Gruffudd, Tywysog Cymru (University of Wales Press, 1986), the most detailed history of the reign of Llywelyn ap Gruffudd, a period which saw him leading several campaigns in the area claimed as a kingdom of Cynllibiwg. The word is not mentioned in contemporary or near-contemporary sources such as the Brut y Tywysogion or the works of Gerald of Wales.