Plaid's Adam Price defeated the former Labour MP, Dr Alan Williams polling 16,130 votes against 13,450 in an 8% swing.
And in the final Welsh result announced at 0505 BST the Conservative's final hope of gaining a seat in Wales was dashed.
Roger Evans, the Tory candidate lost by some 384 votes in the English border seat of Monmouth, to Labour's Huw Edwards.
At the end of the declarations for all forty seats in Wales, the parties remained on the same figures as they were before the election.
Labour had 34, Plaid Cymru four, and the Liberal Democrats two seats.
For Plaid, the Carmarthen East win provided some consolation after the party earlier lost its north Wales seat of Anglesey (Ynys Mon) and failed to capitalise on its Welsh Assembly seats in the south Wales Valleys.
It was also a triumph for the Welsh nationalists who returned their first Westminster MP Gwynfor Evans in Carmarthen in 1966.
" It has been a long hard road but Labour has come home in Ynys Mon "
After one of the first Welsh seats to declare, Plaid members were left reeling with the news that Anglesey - the former seat of their party leader Ieuan Wyn Jones - had been lost.
In an election night marked by a huge drop in turnouts across Wales as across the UK, Labour's candidate Albert Owen won with an 800 majority in the north Wales island seat over the Plaid Cymru.
Ynys Mon was previously held by the Plaid Cymru leader Ieuan Wyn Jones who stood down at this election.
Mr Owen polled 11,906 votes with a two per cent swing, while the Conservatives were third.
A volatile constituency, Ynys Mon had previously been held by all four main parties since World War Two.
Afterwards, Mr Owen said he had been quietly confident of taking the seat.
"It has been a long hard road but Labour has come home in Ynys Mon," he said.
In another close contest the Liberal Democrats retained their rural seat of Brecon and Radnor.
Roger Williams polled 13,073 - just 751 votes more than the Conservative Felix Aubel who engineered a nine per cent swing.
The Conservatives had fought hard in this large rural constituency in the wake of the foot-and-mouth crisis.
One of Labour's 1997 women winners, Betty Williams, was returned to Westminster in Conwy with a majority of more than 6,000 over the Conservatives.
This was Plaid Cymru's second target seat but their candidate only came in fourth.
The Conservatives failed at another target seats - Clwyd West - where Labour's Gareth Thomas was returned to Westminster.
Meanwhile, the Liberal Democrat's Welsh campaign leader Lembit Opik held on to their second rural seat of Montgomeryshire with the Conservatives again in second place.
But the party failed by just 659 votes to win their target seat of Cardiff Central as Labour's Jon Owen Jones polled 13,451 to retain the seat.
The Lib Dems had fought hard in student dominated Cardiff Central - their main target seat in Wales.
Plaid Cymru meanwhile held onto their three other seats in the west and north, but failed to make any major inroad in its target south Wales Valleys seats.
Labour swept to victory in their traditional heartland of Rhondda
Chris Bryant had a majority of more than 6,000 over Plaid's Leanne Wood.
"In the end the people of the Rhondda made their minds up on who was going to get the best deal for the Rhondda," said Mr Bryant.
Former Euro MP Wayne David was returned in Caerphilly with a reduced majority of 22,597, while Plaid's candidate Lindsay Whittle was in second place with 8,172 and a swing of seven per cent.
Mr David fought the seat vacated by the former Welsh Secretary Ron Davies who has stepped down to concentrate on his work as a member of the Welsh Assembly.
In the nearby Cynon Valley, Labour's Ann Clwyd was also returned with 17,685 votes, with Plaid Cymru second.
And in another south Wales Valleys seat - Islwyn - Labour were again triumphant with Don Touhig returned as MP and the Liberal Democrats were in second place.
Even in Llanelli where Plaid Cymru managed a 12 per cent swing, Labour were unassailable with former shadow Defence Secretary Denzil Davies being returned.
Former First Minister in the Assembly and former Welsh Secretary, Alun Michael, was comfortably returned in his seat of Cardiff South and Penarth,
Welsh Office Minister David Hanson won again in Delyn, and Trade and Industry Minister Peter Hain was returned in Neath while long-serving Labour backbencher Paul Flynn held onto Newport West.
Labour also held on to Aberavon, the former seat of Sir John Morris, the former Attorney General and longest serving MP in Wales.
Hywel Williams won the seat with 19,603 votes and Plaid Cymru were in second place.
Elsewhere, Labour has held onto the first three seats to declare in Wales - Wrexham, Clwyd South and the Vale of Clwyd.
Martyn Jones has won Clwyd South with 17,217 votes - a reduced majority from the 1987 election on a smaller turnout.
Plaid Cymru was third and the Conservatives who recorded a swing of more than four per cent were in second place.
Chris Ruane has also been returned in the Vale of Clwyd with 16, 179 votes - again with a reduced majority on a lower turnout.
Earlier still at 12I4 BST in Wrexham Ian Lucas became the first MP to be returned from Wales to Westminster.
He polled 15,934 votes. The Conservatives were in second place with 6,746 and the Liberal Democrats third with 5153.