Llanwddyn borders the county of Gwynedd to the northeast, with the Powys communities of Llangynog and Pen-y-Bont-Fawr to the northwest, Llanfihangel-yng-Ngwynfa to the south east and Banwy to the southwest.

[4] The main feature of the community is the 4.54 square kilometres (1.75 sq mi) reservoir, which drowned the original village when it was created in the 1880s.

[7] In 1873 the local vicar, Reverend Thomas H. Evans, published a mass of information about Llanwddyn in Volume VI of the Montgomeryshire Collections.

[7] In 1877 the expanding English city of Liverpool identified the Vyrnwy Valley as a suitable site for a reservoir to supply fresh water to its citizens.

It included a church, dedicated to St Wddyn, which was consecrated on 27 November 1888, the day before the valve of the dam was finally closed.

Sebald's 2001 novel Austerlitz; the titular character is disturbed and affected by his imagining of the drowned village after being shown the Vynrwy reservoir by his adoptive father, who was born there.