It is famous in Welsh literary history as home of one of the most important collections of Welsh manuscripts. These manuscripts came into the possession of the collector and antiquarian Robert Vaughan, who lived at Hengwrt during the 17th century. They included treasures such as the White Book of Rhydderch, Black Book of Carmarthen, Book of Taliesin and the Book of Aneirin, along with the Brut y Tywysogion. The collection includes an important early copy of Chaucer also known as the "Hengwrt Chaucer" or, misleadingly, the "Hengwrt Manuscript".
These manuscripts remained safe in the Hengwrt library for 300 years. Edward Lhuyd is recorded as having seen the antiquarian books there in 1696. A number of 18th and early 19th century antiquarians visited the mansion to see and copy manuscripts, including Evan Evans, William Owen Pughe and Iolo Morganwg. The library was inherited by William Watkin Edward Wynne of Peniarth in 1859, and subsequently John Williams purchased the library and donated it to the new National Library of Wales in Aberystwyth.
The room in which the manuscripts are displayed today is named "Hengwrt", after the mansion.
- "Hengwrt Chaucer" on the National Library of Wales website