Labour is clear favourite to win in Batley and Spen, Yorkshire, where former Coronation Street actress Tracy Brabin is standing as the party's candidate seeking to carry on the work of Mrs Cox.
Yet far-right groups the British National Party, the National Front and Liberty GB all fielded candidates.
She said: "Tonight is a bittersweet occasion for me. That this by-election has had to take place at all is a tragedy", she said.
He said: "This was Jo at the general election count past year as I briefed her on the numbers. We have shown that we stand together with one voice choosing unity and hope".
Mrs Cox's widower Brendan congratulated Ms Brabin on Twitter.
Of the nine other parties who stood, none managed to reach the 5% threshold.
Turnout was just 26% - one of the lowest by-election turnouts since the Second World War.
Counting is under way in by-elections to pick successors to David Cameron and Jo Cox, the Labour MP who was killed in her constituency just days before the European Union referendum.
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He said yes, and Heather set to work, dedicating hours crafting the head of baby Jesus out of orange clay. A temporary head on a statue of Jesus in Sudbury , Ont. has garnered worldwide attention.
"But it is important for me too to build on Jo's legacy. and to bring my own special experience to Batley and Spen".
"She (Ms Brabin) was giving the impression she had a mandate but, of course, if you have an uncontested election you don't get a mandate".
He told a rally of supporters: "We are going to miss her, but I know that everyone is proud of you for stepping into her shoes". He also tweeted a picture of he and his wife during the 2015 general election.
The Liberal Democrats are represented by businesswoman Liz Leffman, and Ukip by Dickie Bird, in the Oxfordshire seat.
He said: "This was Jo at the general election count a year ago as I briefed her on the numbers".
A jubilant Lib Dem leader Tim Farron said the result was a rejection of the Tories' plans for a "hard Brexit" while marking a return to the "political big time" for his party after their disastrous general election performance.
Elsewhere around the country, the Conservatives held on to David Cameron's old parliamentary seat.