It is the smaller of two lakes within this mountain massif: the slightly larger Llyn y Fan Fawr is about 2 miles (3.2 km) to the east.

The northern face of Waun Lefrith was home to a glacier during the ice ages which gouged out the cwm in which Llyn y Fan Fach now sits.

[2] Large moraines occur to the east of the summit at the base of the scarp, and below the prominent peak of Picws Du as well as those damming the Lake.

In the folk tale,[3] a local young man, son of a widow from Blaen Sawdde (near Llanddeusant) agreed to marry a beautiful girl who arose from the lake, with the condition that he would not hit her three times.

He complied easily because the girl was so beautiful, and they were happy for years putting up a house in Esgair Llaethdy near Myddfai, and bringing up a family there.

The lake was enlarged by the construction of a dam during World War I to provide an additional water supply to Llanelli, the population of which had grown by 34% between 1901 and 1911.

Labour was initially provided by some 175 navvies but harsh working conditions resulted in their departure and replacement with over 150 conscientious objectors who lived in barracks (at the site of the modern car park) and with local families.

A path continues to the west up the slope to the escarpment, following the edge of the cliff, reaching the summits of Waun Lefrith and then Picws Du, both of which overlook the lake.

The red kite was previously restricted to this and adjoining areas in South Wales such as Mynydd Mallaen, mainly as a result of persecution by farmers and gamekeepers, but attitudes have now changed.