[1][2] Born at 'Glandenys', Silian, near Lampeter, where he also died, Cayo-Evans was educated at the independent, co-educational Millfield School in the village of Street in Somerset, England.

[2] His father was John Cayo Evans, a professor of mathematics at St David's College, Lampeter and High Sheriff of Cardiganshire in the period 1941–42.

[2] Best known as a leader of the Free Wales Army, Cayo-Evans appears to have become inspired during the early 1960s, especially during the building of the Tryweryn reservoir.

Security services' files from the UK National Archives described Cayo-Evans having "a mental age of 12", and Coslett, his second-in-command, as "unbalanced", although both of these claims are disputed.

[4] In April 2002 Sgript Cymru produced a play by Dic Edwards based on Julian Cayo-Evans.