According to the Liber Landavensis, lands at Mathern and for several miles around, as far east as the River Wye, were given to the Bishops of Llandaff by Meurig, in memory of his father Tewdrig, king of Gwent and Glywysing.

Tewdrig had been wounded in a battle with the Saxons near Tintern, perhaps around the year 630, and died at Mathern; the parish church of St. Tewdric was built on the spot.

It is thought that the location – some 28 miles (45 km) east of Llandaff – was chosen partly because its proximity to a well-used crossing point of the Severn estuary, and after their construction the castles at Caldicot and Chepstow gave protection against attacks from the Welsh.

The house was repaired after the death of bishop Roger Craddock in 1382,[1] and after Owain Glyndŵr's rebellion in the early 15th century, in which the other two palaces were destroyed, it was the only one kept habitable.

In 1882, local historian Octavius Morgan described three carved stones, showing symbols of the Holy Trinity, which once formed part of a grand gateway to the Palace dating from the time of de la Zouch; these had been deposited by Lord Tredegar at the museum at Caerleon.

The last major renovations of the building, until the late 19th century, were undertaken by Francis Godwin, bishop between 1601 and 1617, who provided new windows in the west wing.

At Mathern, Tipping noted that:[6] What remained of the old palace, after the lead had been stripped from the greater part of its roofs, and its interior woodwork and fittings had been destroyed or removed, [had been] turned into a farmhouse.

He also laid out a new and largely informal garden around the buildings, in the Arts and Crafts style influenced by the writings of William Robinson and Gertrude Jekyll.

[2] In 1923 it was bought by Col. D. J. C. McNabb,[1] whose widow remained there until it was sold in 1957 to steel makers Richard Thomas and Baldwins, the owners of the Llanwern steelworks 12 miles (19 km) away, for use as a guest house.