The town takes its name from the cantref of Emlyn, an administrative district in medieval Dyfed.^[3]

Newcastle Emlyn Castle, was first mentioned in Brut y Tywysogion in 1215, when it was seized by Llewelyn the Great (Welsh: Llywelyn Fawr).^[4] It was captured by the Welsh during the revolt of 1287–1288 and also by Owain Glyndŵr in 1403.^[5]

The population in 1841 was under 1,000.^[6] Cawdor Hall was completed in 1892.^[7]

The Teifi Valley Railway is nearby, although the town has not had a passenger train service since 1952.^[8]

Newcastle Emlyn hosted the Urdd National Eisteddfod in 1981.^[9]

There are two tiers of local government covering Newcastle Emlyn, at community (town) and county level: Newcastle Emlyn Town Council and Carmarthenshire County Council. The town council meets at Cawdor Hall on Market Square.^[10]

Newcastle Emlyn was historically part of the ancient parish of Cenarth. When elected parish and district councils were established in 1894, Cenarth was given a parish council and included in the Newcastle Emlyn Rural District. In 1897 part of the parish on Cenarth was converted into an urban district called Newcastle Emlyn.^[11] Newcastle Emlyn Urban District was abolished in 1974, with its area becoming a community instead. District-level functions passed to Carmarthen District Council, which was in turn replaced by Carmarthenshire County Council in 1996.^[12][13]

In 1932, the former Co-operative creamery was reopened by Dried Milk Products to make cheese. After a new parent firm, Unigate, decided to sell off its non-milk related dairies, it was bought by the Milk Marketing Board in 1979, but closed again in 1983.^[14]

In 1895, the Teifi Valley Railway of the Great Western Railway (GWR) reached Newcastle Emlyn railway station.^[15] Originally conceived as a 7 ft, ¼ inch broad-gauge line between Carmarthen and Cardigan by the Carmarthen and Cardigan Railway, it was absorbed into the GWR, which developed the line only as far as Newcastle Emlyn.

Passenger services ceased in 1952, but goods services continued until 1973, due in part to milk train services to the cheese-producing creamery.^[14] After the goods service ceased, the lines were removed and the station demolished.^[16]

According to the United Kingdom census 2011 Newcastle Emlyn had a population of 1,883, including Adpar on the Ceredigion side of the River Teifi. A 2017 population estimate put it at 1,888, of whom 52 per cent were female and 48 per cent male, with 379 aged 0–17 years, 979 aged 18–64, and 530 aged over 65.

The 2001 UK census had 69 per cent of the 950 people then living in Newcastle Emlyn speaking fluent Welsh, although the proportion fell in the next decade to 54 per cent, as the town population increased to 1,138 aged 3 or over by 2011.^[17] The drop in Welsh usage in Newcastle Emlyn between 2001 and 2011 was among the biggest in Wales, though not uncommon across Ceredigion and Carmarthenshire.

The latest Estyn inspection report in 2012 on the town's English-medium secondary school notes that only 12 per cent of pupils came from homes where Welsh is spoken, with 31 per cent considered fluent in the language. Parents have the option of sending their children to a designated Welsh-medium secondary school, Ysgol Dyffryn Teifi in Llandysul, Ceredigion. Only 64.8 per cent of the town's residents were born in Wales.

The town has a dual-language primary school,^[18] and also a pre-school establishment known as Meithrinfa Teifi Tots Nursery.^[19]

Newcastle Emlyn has a secondary school, Ysgol Gyfun Emlyn. The town's attractions include an art gallery, the Attic Theatre company and the National Woollen Museum.^[20]

Newcastle Emlyn has association football and rugby teams. Newcastle Emlyn Football Club are members of the Football Association of Wales and Newcastle Emlyn RFC of the Welsh Rugby Union.

A legend of the Wyvern of Newcastle Emlyn (Gwiber Castell Newydd Emlyn) tells how on a fair day when the town was full, a winged wyvern breathing fire and smoke landed on the castle walls, stared threateningly, then settled down to sleep. The general terror gave way to an effort by a few townsfolk to destroy it. A soldier waded the Teifi to a vantage point on the castle side and released a red cloak into the river. The creature, suddenly woken, caught sight of the cloak, fell on it with shrieks and tore it to shreds, but was shot in its vulnerable underparts. The assailant escaped to safety. The dying wyvern turned over and floated down the river, its wound gushing venom that fouled the water and killed all the fish. There was joy at the monster's death.^[21]

  Plonévez-Porzay, Brittany, France

In birth order:

• Evan Herber Evans (1836–1896), Congregational minister
• Allen Raine (1836–1908), novelist
• Peter Rees Jones (1843–1905), founder of Peter Jones department store
• Owen Picton Davies (1872–1940), Liberal Party politician and hotel proprietor
• Martyn Lloyd-Jones (1899–1981), Evangelical leader, buried in the town
• John Elwyn (1916–1997), British painter, illustrator and educator, Adpar
• Dill Jones (1923–1984), jazz stride pianist
• Helen Thomas (1966–1989), peace campaigner at Greenham Common
• Josh Turnbull (born 1988) Welsh rugby union international from Newcastle Emlyn RFC
• Scott Williams (born 1990), Welsh rugby union international from Newcastle Emlyn RFC
• Gareth Davies (born 1990), Welsh rugby union and British and Irish Lion international from Newcastle Emlyn RFC

• Local history of Newcastle Emlyn