Stone tools recovered from Coygan Cave near Laugharne show that hominins, probably Neanderthals, were living in what is now Carmarthenshire at least 40,000 years ago. Modern human habitation began after the Younger Dryas, roughly 11,500 years ago. By the time the Romans came in the first century AD, the land belonged to the Demetae, a Celtic tribe whose chief settlement, Moridunum, sat on a low bluff above the River Towy. The Romans built a fort there around 75 AD and, nearby, one of only seven Roman amphitheatres in Britain. They called the place Moridunum, 'sea fort.' The Welsh still know it as Caerfyrddin and the English as Carmarthen, and there is a strong argument that no town in Wales has been continuously occupied longer.

The county climbs from the muddy estuaries at sea level to Fan Foel, 781 metres, a subsidiary summit of Fan Brycheiniog rising over the moorland of the Black Mountain. Three rivers shape the inhabited country between the heights and the sea. The Towy is the longest and the broadest, looping southwest through Llandovery, Llandeilo, and Carmarthen before spilling into the bay. The Cothi joins it from the north, draining the high country around the Roman gold mines at Dolaucothi where, briefly in the second century, the empire dug for bullion. The Teifi forms the long northern border with Ceredigion, and the valley towns along it carry their populations on both sides of the river. The Loughor, in the east, marks the old boundary with Glamorgan. The Gwendraeth, Tywi, and Taf estuaries cut deep into the south coast, leaving Llansteffan and Laugharne on a peninsula reached by a single road from St Clears.

Norman castles studded the county after Edward I's conquest, and the ruins of the better ones - Carreg Cennen on its crag near Llandeilo, Dinefwr in its deer park, Kidwelly on the Gwendraeth, Llansteffan on the headland over the Towy estuary, and Laugharne on the Taf - still anchor the landscape. The Premonstratensian Talley Abbey was wrecked at the Dissolution but its tall west tower stands. At Laugharne, in a small writing shed on the cliff above the boathouse, Dylan Thomas wrote much of the work for which he is remembered. He called the town 'a timeless, beautiful, barmy town,' and is buried in the churchyard at St Martin's. The boathouse is a museum now. The shed is still there, with the green door closed, looking out over the Taf estuary as if its occupant might be back at any moment.

The county nickname is not idle. Dairy farms cover the river valleys, and the Welsh black cattle and Welsh mountain sheep cover the hills. The National Botanic Garden of Wales, opened in 2000 near Llanarthney, sits inside the parkland of an 18th-century estate and is built around a great glasshouse designed by Norman Foster. Aberglasney's restored Cloister Garden, just up the road, may be one of the oldest surviving in Britain. The Millennium Coastal Park, a ten-mile stretch of reclaimed waterfront west of Llanelli, replaced the old coal-loading docks and tinplate works with paths, ponds, and the Wetlands Centre. The county was one of the worst-performing economic regions in the UK in the mid-2010s, but the recovery, when it has come, has come through this: agri-food, tourism, the careful working of land and coast.

In 1911, 84.9 per cent of Carmarthenshire's population spoke Welsh, and one in five spoke only Welsh. The percentages slid through the twentieth century, the way they slid everywhere in Wales: 82.3 in 1931, 75.2 in 1951, 50.3 in 2001. The 2011 census recorded 43.9 per cent, the first time Welsh was a minority language in the county. The 2021 census dropped it to 39.9 per cent - the largest single-decade decline anywhere in Wales. But 60 per cent of children aged 5 to 14 in the county still speak Welsh, a figure that rose between censuses, and the bilingual schools are full. The language has been pronounced lost before, and it has refused. In the towns - Llanelli, Carmarthen, Ammanford, Llandeilo, Llandovery - and on the radio of Sir Gar, you still hear it spoken in shops, in pubs, on the pavement.

The south coast of the county is more interesting than the road map suggests. Pembrey Country Park covers an old Royal Ordnance factory and the dunes behind eight miles of sand. The Pembrey motor racing circuit, opened in 1989 on a wartime airfield, is the home of Welsh motorsport. Ffos Las racecourse, opened in 2009 on the site of a closed opencast coal mine, was the first new racecourse built in the UK in eighty years. The Wales Coast Path, opened in 2012, runs continuously around the entire Welsh coastline, and in Carmarthenshire it threads through villages and salt marshes and across the long sand spits at Pendine and Cefn Sidan. From a slow aircraft on a clear day, the whole county is laid out in greens and tidal silvers, with Carmarthen at the centre and the mountains in the distance, and from that height it is easy to see why the Welsh called it a garden.