It consisted of the then local authority areas of the Municipal Borough of Llanelly; the Urban Districts of Ammanford, Burry Port and Cwmamman; the Rural Districts of Lanelly and part of Llandilofawr (namely the civil parishes of Betws, Llandybie and Quarter Bach, and Ward I of the civil parish of Llandilo Rural)[3] The division bordered Carmarthen to the west and north, Brecon and Radnor to the north east, Neath to the east, Gower to the south east and the sea to the south.

Llanelly no longer bordered Brecon and Radnor and Gower was extended north and took over the part of the 1918 Neath constituency that had previously adjoined Carmarthenshire.

The constituency area continued to include the same local authorities as in 1918 (apart from a spelling change to Llandilo for the part RDC included):[4] At the 1970 general election the official spelling of the constituency name was altered to Llanelli.

2010 After the United Kingdom general election in May 2010, the constituency has comprised the Carmarthenshire County electoral divisions of Bigyn, Burry Port, Bynea, Dafen, Elli, Felinfoel, Glanymor, Glyn, Hendy, Hengoed, Kidwelly, Llangennech, Llannon, Lliedi, Llwynhendy, Pembrey, Pontyberem, Swiss Valley, Trimsaran, Tycroes and Tyisha.

It was represented by one-time deputy leader of the Labour Party, Jim Griffiths, from 1936 until his retirement in 1970.