Anglesey has a human history far older than the Roman conquest. Its prehistoric monuments and later settlement show that the island was an important part of north-west Wales long before written accounts begin. In Welsh it is Ynys Môn, while Roman writers used Mona. The island's position behind the Menai Strait gave it a degree of separation from the mainland without isolating it from the communities of north Wales. Roman literary sources later made Mona famous because of its connection with resistance to Roman rule and with the Druids.

Tacitus gives the most vivid surviving account of Suetonius Paulinus's assault in 60 or 61 CE. He says Mona had a substantial population and served as a haven for refugees. When the Romans crossed, they faced armed Britons while women in deathly black, with dishevelled hair, moved among the ranks carrying torches. Druids stood around them with raised hands, calling down curses. The spectacle briefly shocked the Roman soldiers before they attacked. Tacitus says the Romans then installed a garrison and destroyed groves consecrated to local religious practice. The familiar modern image of white-robed Druids is not part of Tacitus's description of this battle.

Paulinus could not consolidate the campaign because news arrived of the revolt led by Boudicca in eastern Britain. He withdrew to deal with the wider crisis. Tacitus later describes another campaign against Mona under Agricola in 77 CE. After defeating the Ordovices, Agricola lacked a prepared fleet, so selected auxiliaries familiar with fords and swimming crossed unexpectedly. The islanders, who had expected a formal naval attack, sought peace and surrendered. These Roman narratives are written from the conquerors' perspective, but they establish Mona as an important theatre in the Roman conquest of Wales.

Anglesey also belongs to the mythic geography of the Mabinogi. In the Second Branch, Branwen, sister of Bran and wife of the Irish king Matholwch, suffers humiliation in Ireland after a diplomatic marriage collapses. Bran leads an expedition to rescue her, and the conflict destroys much of both sides. Returning towards Wales, Branwen looks upon the devastation and dies of grief. The tale says she is buried in a four-sided grave on the banks of the Alaw. The monument known as Bedd Branwen is a real Bronze Age burial site whose archaeology is much earlier than the medieval story, but the association gives the landscape a powerful meeting point between prehistory and legend.

Medieval writers also celebrated Anglesey for its agricultural richness. Gerald of Wales used the phrase Môn Mam Cymru, commonly rendered Mother of Wales, reflecting the island's reputation for producing grain and supporting the wider region. That combination of real landscape and layered memory is central to Anglesey's place in Mythopouch: it is simultaneously a historical island, the Mona of Roman military texts and the Ynys Môn of Welsh language and story. Its Historicity is therefore Historical even though many traditions attached to it are mythological or literary.