Colm Murphy and Seamus Daly were alleged to have played central roles in the Real IRA bomb attack in August 1998.
At Belfast High Court on Wednesday, Mr Justice Gillen said the evidence against the pair was overwhelming.
He spoke at the conclusion of a retrial against the men, who previously successfully appealed a finding of liability against them in the original case in 2009.
The Real IRA bomb ripped through the Co Tyrone town in 1998, killing 29 people, including a woman who was pregnant with twins. It also injured more than 200 people, and the attack has become known as the worst atrocity of the Troubles
No one has ever been convicted of the bombing in a criminal court, but three years ago Murphy, from Dundalk, Co Louth and Daly, a bricklayer from Cullaville in Co Monaghan, were held responsible in the initial civil action taken by some of the bereaved families.
I have determined that both defendants were involved in assisting the preparation, planting and detonation of the bomb.
Mr Justice Gillen
Along with Real IRA chief Michael McKevitt and Co Louth republican Liam Campbell, they were ordered to pay out £1.6m in damages.
McKevitt is currently behind bars in the Republic for directing terrorism, and Campbell recently fought extradition proceedings to Lithuania on arms smuggling charges.
They both failed in their bids to overturn the civil judgement and have taken their case to the European Court of Human Rights.
Michael Gallagher's only son Aidan died in the bomb, aged 21.
Speaking outside the High Court on Wednesday, Mr Gallagher said they were delighted with the judgement delivered.
"All the families through the years have stood together and we've held people accountable for what happened at Omagh," he said.
But, he added, nine years after the first case, the families will once again seek a meeting with the Chief Constable, the Director of Public Prosecutions, and their counterparts in the Republic of Ireland.
It was left up to us as families to bring the people before the courts. I think that's wrong.
"We are going to ask the PSNI and the DPP to look at this judgement and to say is there any way that they can bring the people responsible for the Omagh bombing to court, just as we have done."
"Thirty-one people died at Omagh and no one has served a day for that crime."
Beside him, Stanley McCombe, whose wife Anne was killed, said the families were determined to ensure the four men pay out the £1.6m in damages.
"When you go before a court and you are fined for whatever you do, you pay your fine, you pay your debt to society, whatever you do, so why should these people get away, why should these people live in the lap of luxury?" he said.
Murphy and Daly refused to give evidence during the retrial and neither appeared in court for Wednesday's judgement.
The judge said the case against Murphy and Daly was "overwhelming", adding that he had drawn a negative inference from their failure to provide any explanation in court.
Much of the evidence against them centred around a mobile phone, which lawyers for the victims' families believed was supplied to the bomb team by Murphy.
Their case also stated that a phone registered to the builder could be traced to within 5km of the town an hour before the blast.
It was also claimed in the new case that Daly supplied mobile phones to the bomb team.
The barrier of time has not served to disguise the enormity of this crime, the wickedness of its perpetrators and the grief of those who must bear its consequences.
Mr Justice Gillen
In November 2000 Daly pleaded guilty and was convicted of Real IRA membership, and Mr Justice Gillen said this was relevant "to determining the issue of liability in this case".
"Even 15 years on nothing can dilute the pulsing horror of what happened," said the judge, who added that anyone who knowingly provided mobile phones to the bomb team was liable.
He described Murphy's explanation for failing to give evidence as being due to his lack of confidence in the judicial system as "bordering on the risible".
The judge confirmed: "It makes the prima facie case even stronger and renders it now overwhelming."
"Given the strength and quality of the evidence, I have determined that both defendants were involved in assisting the preparation, planting and detonation of the bomb in circumstances where those involved in assisting those acts would be joint tortfeasors (individuals who committed a wrongful act injuring another person)," he said.
The defendants have had their full measure of legal aid and the ability to participate in this trial. Let it come to an end.
Lord Brennan QC, for the victims' families, said the verdicts were "damning" and left no room for appeal.
The barrister also confirmed plans to pursue all four men now held liable for the bombing.
"The plaintiffs intend to proceed to enforce their judgment against all defendants, these two as well.
"There is no reason why that entitlement should be further delayed. They wish me to assure the court that enforcement will be with vigour here and in any relevant jurisdiction," he added.