Castle and Marcher lordship in southeast Wales, partly in the valley of the Usk between Brecon and Abergavenny and partly lying between the Usk and the Wye. Two routes linked the valleys of the Usk and Wye. One of them lay along the valley of the River Llyfni: the name of the castle meant the head of the valley of the Llyfni.
The lordship included most of the old Welsh cantref of Talgarth, and was originally part of the lordship of Brecon. It was held by several families, eventually becoming part of the Mortimer lands after the fall of Hugh Despenser the Younger, who had held it 1322-26. The Mortimer tenure was interrupted by the attainder and execution of the first Earl of March in 1330, Blaenllyfni passing to the Talbots until 1354. From the Mortimers it passed by inheritance to Richard, Duke of York in 1425, and so in 1461 to the Crown.
In 1536 Blaenllyfni became the east of Breconshire.