The reservoir is fed by two main streams, the Afon Frogwy entering from the west and the Afon Erddreiniog from the north east end. With a maximum length of and a total water surface area of , Llyn Cefni is the second largest body of water on the island after Llyn Alaw, also a reservoir. There is a dam on the south-east edge of the reservoir, next to which is a car park. The northeastern end of the reservoir and the immediate surrounding area have been designated as a nature reserve, and as a further measure of conservation, a plantation of trees runs along its north shore. The tracks of the Anglesey Central Railway, although no longer used, still run over an embankment which crosses the water. The reservoir is easily accessible from Llangefni, as well defined paths run from the town through the local nature reserve, The Dingle, one of the few ancient woodland sites on the island. It can also be accessed by the Lôn Las Cefni cycleway which follows the Afon Cefni from The Dingle and divides at the dam.