The Unitarian cause began in Llwynrhydowen in 1726, where Jenkin Jones is said to have preached to the first Arminian congregation in Wales which sprang from Pantycreuddyn church. The first chapel was built in 1733, and after Jenkin Jones death in 1742 he was followed by his nephew, the Rev. D Lloyd of Brynllefrith. He was a very popular preacher and the congregation increased enormously under his care making it necessary to extend the chapel in 1745. This was completely rebuilt in 1791 on a slightly different site, the lease being given for a term of 99 years for a rent of 12 pence a year by Mr D Lloyd of Alltyrodyn.
In 1834 the present, third, chapel was built on the same site as the second, and the first burial in the graveyard took place. The chapel was again renewed at a cost of £300 in 1862. Sittings as given in the Religious Census of 1851 are free 42, other "none", and an average attendance of 4-600 is given.

The building ceased to be used as a chapel after the "Troad Allan" of 1876, when the congregation was evicted from the building by John Lloyd of Alltyrodyn who claimed that the conditions of the lease had been breached. The congregation built a new Llwynrhydowen Chapel (NPRN 7289), but on the death of Mr Lloyd two years later, his sister Mrs Massey returned the building to the trustees. It was subsequently used as the Sunday School, and also for village Christmas concerts and eisteddfodau until ca. 1959 and the construction of the Neuadd Goffa D.H.Evans in Pontsaen. The building is currently in the care of the Welsh Religious Buildings Trust.

The chapel is a long-wall entry type built of coursed rubble stone with dressings and quoins of paler ashlar stone and a half-hipped slate roof. There are two doorways to the outer bays, with paneled doors and fanlights with intersecting tracery. There is a central pair of tall round-headed windows with sash glazing beneath a head of intersecting tracery, and two shorter, similar windows to either end over the doorways. In the centre is a slate plaque inscribed "Llwynrhydowain 1834". There are two inscriptions from the earlier phases of building and four 19th century memorial stones, including one to Mary Thomas, the first wife of Gwilym Marles.

In the interior are slate flagged vestibules, leading ahead up steps to the main chapel interior and to gallery stairs. Each vestibule has a 19th century half-glazed screen wall parallel to pulpit and 2 doors leading to chapel, fitted with etched and coloured-glass margin panes. The main interior has a wooden floor and white plaster walls and a ceiling with a circular centre panel and moulded coving. There are bench seats to the ground floor, laid out in three blocks to the rectilinear Sedd Fawr. Two flights of steps leading up to the rectilinear platform pulpit have turned bobbin balusters of 17th century style. The pulpit has a central canted projection with moulded panels and a sloping lecturn. in the NE corner is the former ministers library containing a two tier bookcase with a zinc front. There is a mid 19th century gallery to three sides, supported by 5 iron columns stamped "T BRIGHT CARMARTHEN" and with a front of grained and moulded panels. Opposite the pulpit is the clock with the ledgend "Dd Jones, Lampeter". The gallery is fitted with open bench seats.

Hen Gapel is Grade II Listed for its historical importance in the history of Utanitarianism in Wales and its unrestored late Georgian character. A Unitarian Museum was established here briefly from 1976, the building is now in the care of the Welsh Religious Buildings Trust.

RCAHMW, March 2014
Aubrey J. Martin, Hanes Llwynrhydowen (Llandysul: 1977)
D. Elwyn Davies, Y Smotiau Duon (Llandysul: 1981)
Elfyn Scourfield, Carmarthen craftsmen and implement makers, The Carmarthenshire Antiquary XXVII (1991).