Sinn Fein's Francie Molloy has won the Mid Ulster by-election to fill the vacant seat at Westminster.
The outgoing principal deputy speaker at the Northern Ireland Assembly succeeds party colleague and Stormont deputy First Minister Martin McGuinness.
"I would like to thank my colleague and comrade Martin McGuinness for the work he has done this past 15 years in moving Mid Ulster forward," said Mr Molloy after the results were declared at Cookstown Leisure Centre.
"Martin is going to be a hard act to follow, I understand that, but we will try our best to do that and we will continue to build the process and work with everyone - I want to represent all the people of Mid Ulster, not just those who voted for me, not just the Sinn Fein support."
He added: "We want to get planning permission for factories in rural parts of Mid Ulster and ensure those jobs can be provided across this constituency."
In keeping with Sinn Fein's abstentionist policy, Mr Molloy will not take his seat in the House of Commons.
Sinn Fein saw its majority substantially cut from 15,363 at the 2010 General Election to 4,681.
A lower turnout - down from 63.23% in 2010 to 55.38% - and the emergence of a unified unionist candidate were undoubted factors in the reduction.
However the Sinn Fein share of the vote also reduced from 2010 (52.00% to 46.93%) while other parties made some in-roads against its dominant position.
Mr Molloy secured 17,462 votes to the 12,781 (34.35%) polled by his closest rival, agreed unionist Nigel Lutton.
Of the remaining two candidates, Social Democratic and Labour Party MLA Patsy McGlone polled 6,478 votes and the Alliance Party's Eric Bullick secured 487.
The SDLP's share of vote was up from 14.26% in 2010 to 17.41% this time round while Alliance's also went up slightly (1.31% from 0.97%).
The by-election campaign was played out under the shadow of a historic Troubles murder
The highly emotive contest saw Mr Molloy come face to face with Mr Lutton - the son of a former policeman whose murder the senior republican was alleged in the House of Commons to have had a role in.
Mr Molloy has always vehemently denied the claim levelled against him six years ago under parliamentary privilege by Upper Bann Democratic Unionist MP David Simpson that he was a suspect in the 1979 shooting of ex-Royal Ulster Constabulary reservist Frederick Lutton.
As predicted, there was no handshake between the victorious Sinn Fein politician and Mr Lutton at the declaration, although the unionist candidate did say the men exchanged hellos.
Mr Lutton stood as an independent with the backing of Stormont's main unionist parties, all of whom stood aside in a bid to maximise the pro-Union vote.
The proportion of the vote secured by the Portadown undertaker was up nearly 2% on the combined total achieved by the various unionist candidates in 2010. The combined nationalist vote fell by the around the same degree.
"I would like to thank the voters of Mid Ulster for increasing the Unionist vote," Mr Lutton said after the declaration.
He added: “I think it shows that the parties can work together and that people will come out in support, because unionism needs to stop beating itself.”
But asked if he will now consider running for the Assembly or in next year’s council elections, Mr Lutton declined to comment.
A senior Ulster Unionist said: “It would seem to have worked to the extent that people who have not voted for a while, or have not voted before, have come out this time. But that has to be held against those who did not vote for whatever reason.”
Mr McGuinness resigned from the Westminster seat he first won in 1997 late last year, insisting he wanted to focus on business at the Northern Ireland Assembly.
The emergence of Mr Lutton as the unionist unity candidate had major ramifications for the Ulster Unionist Party (UUP).
In the wake of the UUP's decision to back Mr Lutton, two of its high-profile Assembly Members - Basil McCrea and John McCallister - resigned in protest and have subsequently signalled their intent to form a new party.
Francie Molloy (SF) 17,462 (46.93%, -5.07%)
Nigel Lutton (Ind) 12,781 (34.35%)
Patsy McGlone (SDLP) 6,478 (17.41%, +3.15%)
Eric Bullick (Alliance) 487 (1.31%, +0.34%)
SF maj 4,681 (12.58%)
Electorate 67,192; Turnout 37,208 (55.38%, -7.85%)
2010: SF maj 15,363 (37.62%) - Turnout 40,842 (63.23%)
McGuinness (SF) 21,239 (52.00%); McCrea (DUP) 5,876 (14.39%); Quinn
(SDLP) 5,826 (14.26%); Overend (UCUNF) 4,509 (11.04%); Millar (TUV)
2,995 (7.33%); Butler (Alliance) 397 (0.97%)