Tommy Sheridan has been convicted of perjury following a 12-week trial at the High Court in Glasgow.
The 46-year-old former MSP was found guilty of lying during his successful defamation case against the News of the World newspaper in 2006.
He sued after it printed allegations that he was an adulterer who had attended a swingers' club.
Sheridan has been given bail and faces a prison term when he is sentenced on 26 January.
His wife Gail, also 46, was acquitted last week after the Crown withdrew charges against her.
A couple of female members of the jury were in tears when the verdict was read out.
Following the verdict, Sheridan's solicitor, Amar Anwar, read a statement on his client's behalf outside the court.
In the statement, Mr Sheridan said: "For three years my wife and I have faced charges of perjury. Today, I was convicted and Gail was acquitted of any crime.
"I have fought the power of News International all my political life, and I make no apologies for taking on the might of Rupert Murdoch.
"Several million pounds of public money was spent investigating me and my wife.
"Is it not time that similar resources were devoted to investigating the activities of the News of the World."
After this was read out, Gail Sheridan gave her reaction to the waiting media.
"I would like to thank everybody from across Scotland for their wonderful support throughout the last six years that they've given to Tommy and I," she said.
"Our family and our real friends, our real friends, have stood by us. I have and will always stand by Tommy."
At this point, a highly emotional Sheridan hugged and kissed his wife.
Minutes before, the politician was found guilty of five allegations under one charge on the indictment.
The last remaining allegation, that he had a sexual relationship with former News of the World columnist Anvar Khan, was deleted.
In coming to their verdict, the 14 members of the jury ruled that: Sheridan did tell a meeting of the SSP executive committee on 9 November 2004 that he had visited Cupid's in Manchester in 1996 and 2002 with Ms Khan.
The jury found that party members Keith Baldassara and Alan McCombes did state at the meeting that they had previously raised the issue of Sheridan attending a sex club in Manchester with him and that he admitted that it was true.
It was also found that he did visit Cupid's with Andrew McFarlane, Gary Clark, Ms Khan and Katrine Trolle on 27 September 2002 and that he did have a sexual relationship with Katrine Trolle between 1 January and 31 December 2005.
Following the majority verdict, advocate depute Alex Prentice moved for sentencing, giving a short biography of Sheridan's political career and mentioning previous convictions for breach of the peace.
Lord Bracadale requested background reports and said any submissions by Sheridan could be made at the sentencing hearing next month.
The judge told Sheridan: "You have been convicted of the serious offence of perjury and must return to court expecting to begin a prison sentence."
He excused the jury from service for eight years, telling them it had been a "long and arduous" trial.
In the aftermath of the verdict, some of the main parties involved have been giving their reaction.
Det Ch Sup Malcolm Graham, of Lothian and Borders Police, said the perjury investigation, which Sheridan attacked in court as "a vendetta" had followed on from "the comments of Lord Turnbull, the presiding judge at the civil case in 2006".
"All of the evidence from the civil action brought by Mr Sheridan was considered and inquiries made it clear that perjured evidence had been given during the civil hearing," he said.
In a statement, the Scottish Socialist Party (SSP), which Sheridan formerly led, said the verdict would now define him.
It stated: "By his actions over six years, Tommy Sheridan has disgraced himself and negated his political contribution to the socialist cause over 25 years.
"History will now record that he did more harm to the socialist cause in Scotland than any good he ever did it."
Unsurprisingly, Solidarity, the party Sheridan formed after leaving the SSP, offered its continuing support.
"Tommy Sheridan's only crime has been to speak truth to power for as long as he has been involved in socialist politics," it said.
"He is a working class fighter who has waged war on poverty and injustice where ever he has found it.
"We in Solidarity are proud to call ourselves comrades and friends of Tommy Sheridan."
Sheridan's mother, Alice, said her son and his wife would remain strong.
She added: "I'll cope - that's my son, he's done nothing but good for people all his life."
The verdict, which took six hours to deliver, brings to a close what is thought to be the longest-running perjury case in Scottish legal history.
Its beginning can be traced to Sheridan's successful defamation action against the News of the World in August 2006.
The civil case tore the SSP apart as 11 colleagues testified against Sheridan, saying that he had admitted attending the sex club during a party meeting in 2004.
Sheridan won the case at the Court of Session in Edinburgh, however, and was awarded £200,000 damages.
Two months later, Edinburgh's procurator fiscal ordered Lothian and Borders Police to investigate allegations of perjury during the case.
During the investigation, the News of the World also published details of a video tape, secretly recorded by Sheridan's best man George McNeilage, in which the politician was alleged to have admitted visiting a swingers' club in Manchester.
When the police probe concluded, Sheridan and his wife were charged with perjury.
The resulting trial at the High Court in Glasgow, which got under way in October, saw dozens of witnesses give evidence against the former MSP.
During the second week of the trial, Sheridan released his defence QC Maggie Scott and opted to represent himself - just as he had done successfully during the 2006 defamation action.
Like the civil case again, a number of former SSP colleagues stated that he had admitted the sex club visit at an executive meeting of the party in November 2004.
The trial also heard allegations of affairs with journalist Anvar Khan and former SSP colleague Katrine Trolle.
'Not credible or reliable'
The video tape, which Sheridan's best man George McNeilage sold to the News of the World, also formed a central part of the prosecution case.
In the final weeks of the trial, perjury charges against Gail Sheridan were dropped and she was acquitted.
Prosecutor Alex Prentice QC said it was no longer in the public interest to pursue them.
The number of perjury allegations against Mr Sheridan was also reduced from 18 to six.
As the trial came to a close, the Crown's case centred on three key areas - what Sheridan was alleged to have said at the SSP meeting in November 2004, the alleged visit to the Manchester swingers' club and the secret video tape recorded by Mr McNeilage.
In a marathon closing speech, lasting five hours over two days, Sheridan told the jury that the Crown witnesses were not credible or reliable.
He accused police of conducting "a vendetta" against him during their investigation and claimed the tape was a fabrication designed to incriminate him.
The jury, however, chose to believe the Crown's case, and convicted him of perjury.
The Rise and Lies of Tommy Sheridan will be broadcast on Thursday 23 December at 2100 GMT on BBC One Scotland. It will be available on the BBC iPlayer for a week afterwards.