Anna Turley - who lost her seat in Redcar, in the general election on December 12 - had sued Unite and blogger Stephen Walker.
Mr Justice Nicklin, who oversaw a High Court trial in London in November, ruled in her favour on Thursday.
Ms Turley said a 2017 article on Mr Walker's Skwawkbox blog, which contained a press statement from Unite, libelled her by conveying the meaning that she had acted dishonestly when submitting an application to join the union.
She also said Unite had misused her private information.
Unite bosses and Mr Walker fought the case and said Ms Turley had been dishonest and was not fit to be an MP. They said the article was true or justified in the public interest.
The judge concluded that publication of the article had caused "serious harm" to Ms Turkey's reputation.
He said he rejected allegations that Ms Turley had been dishonest.
"I am very pleased with today's verdict, and to be able to say that I have won my libel action against Unite the Union and Skwawkbox," said Ms Turley after the ruling.
"It gave me no pleasure to undertake this action, but the accusations were so serious and damaging to my reputation that I had no choice but to defend myself through the courts."
Mr Justice Nicklin found Unite and Mr Walker liable.
A Unite spokeswoman said Unite bosses were "very disappointed" with the decision. Lawyers said the union and Mr Walker wanted to appeal.
The article related to a Unite membership application Ms Turley made in December 2016.
Mr Justice Nicklin heard how Ms Turley had applied to be a Unite member under a Community membership category.
He was told that Unite's Community section was aimed at people not in paid employment and cost 50p a week.
A barrister representing Unite said Ms Turley had been willing to "conceal, mislead and deceive".
Anthony Hudson QC said Ms Turley wanted to vote against Unite general secretary Len McCluskey in an election without being noticed and without the union knowing she was an MP.
Ms Turley said the Skwawkbox article made "false and defamatory" allegations about her and impugned her honesty.
"I had not dishonestly joined the Community section of Unite and there was no reason to suspect me of being dishonest," she had told the judge.
"I believed I was entitled to join it. I am not dishonest and have not lied or sought to mislead."
Ms Turley had links to a WhatsApp group of Labour MPs, known as the Birthday Club, opposed to Jeremy Corbyn's leadership, the judge heard.
Mr Hudson said her application flowed from a Birthday Club WhatsApp discussion.
He said the Birthday Club members had come together to oppose Mr Corbyn's leadership.
Ruth Smeeth, who was then Labour MP for Stoke-on-Trent North, had circulated a link to Birthday Club members about a Unite general secretary election.
Mr Hudson suggested that Ms Turley had been part of an attempt to "oust" Mr McCluskey and that the ultimate aim had been to "oust" Mr Corbyn as Labour leader.