Llyn Mymbyr is located in Dyffryn Mymbyr, a valley between the village of Capel Curig and Pen-y-Gwryd hotel in Snowdonia, Wales. Its maximum depth reaches 9.1 m (30 feet). It is fed by the River Nantygwryd, which springs near Llyn Cwm-y-ffynnon, a tributary of the Afon Llugwy. This lake is situated in a valley formed by a ring of sandstone called the Capel Curig Volcanic Formation. Near the lake the fiery rocks of Moel Siabod can be observed. At western part of the lake Garth farm campsite can be found and it offers access to the lake, too.