John Dyer was the fourth of six children born to Robert and Catherine Cocks Dyer in Llanfynydd, Carmarthenshire, five miles from Grongar Hill. His exact birth date is unknown, but the earliest existing record of John Dyer dates his baptism on 13 August 1699 – within fourteen days after his birth as was the tradition of the time – in Llanfynnydd parish. His grandfather was churchwarden there and his father was a highly successful solicitor in Llanfynnydd and owned several properties in the neighbourhood. Presumably for financial opportunity and greater living space for six children, the family moved in 1710 to the mansion of Aberglasney in the nearby parish of Llangathen.