Sinn Féin leader Mary Lou McDonald described her party's win in the West Tyrone by-election as a "stunning victory".
Carrickmore solicitor Órfhlaith Begley topped the poll to become the constituency's first ever female MP.
However, the comfortable 10,000-plus majority secured by predecessor Barry McElduff last June was cut by more than 2,386 as turnout dropped more than 13 percentage points to just over 55 per cent.
The abstentionist party's share of the vote fell four points to 46.7 per cent compared to the last Westminster election, while DUP candidate Tom Buchanan's share dropped by a similar margin to 23.9 per cent.
The outcome in terms of percentage share was better for the SDLP's Daniel McCrossan and Ulster Unionist Chris Smyth, who increased their vote by 4.9 and 3.2 points respectively.
Alliance's Stephen Donnelly increased his share of the vote by 0.9 points – or 130 votes – to 3.2 per cent.
Ms Begley noted that her victory, secured with 16,346 votes, came in the centenary year of the first woman MP elected to Westminster, the Irish nationalist Countess Markievicz.
"This is the first female MP for West Tyrone you are looking at," she told cheering supporters inside the count centre in Omagh.
"So today we have made history, we are the history makers and we will continue to be."
Referencing the circumstances that saw her predecessor quit in January amid a furore around his posting of a video on the anniversary of the Kingsmill massacre, the newly-elected West Tyrone MP said there were "lessons to learn".
"Going forward it's vital for people in public life to recognise and acknowledge the pain that exists," she said.
The new MP also issued a direct message to unionists, urging reconciliation.
"What I am about is building for the future – reaching out to unionism and building bridges between our communities - so that we can live in a shared and reconciled future together," she said.
Mrs McDonald said the result was an endorsement of Sinn Féin and a vindication of the party's desire for "real power-sharing based on equality, based on respect".
Mr Buchanan, who polled 8,390 votes, said he respected the outcome but was disappointed that the constituency would not be represented at Westminster.
"Whilst West Tyrone has an MP it does not have one who is prepared to do the full job expected of them," he said.
SDLP candidate Daniel McCrossan, who came in third with 6,254 votes, also lamented the lack of representation at Westminster.
"Unfortunately yesterday’s result means that Nationalism here in the North has again been left without a voice in the most important chamber whilst the assembly sits dormant," he said.
The party's 18% vote shared was up almost 5%. He was the only candidate not to make a speech after the declaration in the early hours of Friday.
Ulster Unionist Chris Smyth said his increased vote demonstrated that "progressive unionism is very much alive in West Tyrone".
"I’m confident in my unionism, I’ll continue to stand up for the innocent victims of terrorism and I’ll continue to represent the future of unionism," he said.
West Tyrone Result
Órfhlaith Begley (SF) 16,346 (46.66 per cent) -4.06 per cent
Thomas Buchanan (DUP) 8,390 (23.95 per cent) -3.00 per cent
Daniel McCrossan (SDLP) 6,254 (17.85 per cent) +4.90 per cent
Chris Smyth (UUP) 2,909 (8.30 per cent) +3.12 per cent
Stephen Donnelly (Alliance) 1,130 (3.23 per cent) +0.93 per cent
Sinn Féin majority: 7,956
Registered Electorate 64,178;
Turnout 35,029 (54.58 per cent) -13.36 per cent
