The constituency was established in 1918, as a division of Carmarthenshire, located in the south east of the county. This area had, until 1918, been the southern part of the constituency of East Carmarthenshire.
In the next redistribution of constituencies in Wales, which took effect in 1950, the northern boundary of the constituency was slightly altered. 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):
The borough of Llanelly;
The urban districts of Ammanford, Burry Port and Cwmamman;
The rural district of Llanelly and the parish of Bettws in the rural district of Llandilo.
At the 1970 general election the official spelling of the constituency name was altered to Llanelli. This followed the change in name of both the borough and rural district in 1966.
The constituency appears to have been unchanged by the redistribution. The local authorities remained the same (apart from spelling changes):
The borough of Llanelli;
the urban districts of Ammanford, Burry Port and Cwmamman;
the rural district of Llanelli and the parish of Bettws in the rural district of Llandeilo.
The substantial local government changes which took effect in 1974 did not affect this redistribution as it used the boundaries as they existed in November 1970 to construct parliamentary constituencies.
The redistribution altered the constituency by 8.4%. 96.2% of the new constituency had been in the old one. 3.8% of the electors came from the former Carmarthen constituency.
The area now formed part of the new county of Dyfed. The district level local government units contained in the constituency were the Borough of Llanelli and Wards 2–6 and 9 of the Borough of Dinefwr.
In this redistribution the constituency was reduced so that it covered the same area as the Borough of Llanelli.
Llanelli has traditionally been an ultra-safe Labour seat, with a Labour MP representing the constituency since 1922. It was represented by one-time deputy leader of the Labour Party, Jim Griffiths, from 1936 until his retirement in 1970. In recent years however Labour's majority had been somewhat eroded by Plaid Cymru, who as of 2021 have won the equivalent seat in the Senedd in two of the six Senedd elections to date. At the 2015 general election, however, the Labour majority increased once again.
^Llanelli is the spelling Craig uses in his compilation of results from 1950–1973, despite the entry in his book on parliamentary boundaries for the 1950 redistribution following the statute in using Llanelly.
^ abThe Parliamentary Constituencies (Wales) Order 1970 (1970/1675).