Pat Finucane was killed in his home in 1989
Lord Chief Justice Sir Robert Carswell returned a verdict of not guilty in the absence of evidence against William Stobie on Monday.
The self-confessed one time Ulster Defence Association (UDA) quartermaster and ex-RUC informer was accused of aiding and abetting in the murder of Mr Finucane in February 1989 and in the murder of student Adam Lambert.
The UDA is the largest loyalist paramilitary group in Northern Ireland.
Mr Finucane, a high-profile Catholic solicitor, was shot dead by the group in front of his family at his home.
The court heard the director of public prosecutions had concluded that the central witness, former journalist Neil Mulholland, was not cabable of giving evidence.
After the brief hearing on Monday, Mr Stobie said: "I am glad it's all over."
His solicitor Joe Rice said: "Mr Stobie is delighted. He always maintained his innocence."
He also said William Stobie would be considering legal proceedings in terms of the disruption to his life that the case had caused.
'Vindicated'
Meanwhile, a judge of international standing is expected to begin a fresh investigation into Mr Finucane's murder by next April.
The appointment of a judge was agreed at the Weston Park peace process talks in the summer involving the Northern Ireland parties, Tony Blair and his Irish counterpart Bertie Ahern.
But the Finucane family said it was not enough and are calling for a full public inquiry.
Mr Finucane's son, Michael, said he believed there was never any hope of a successful criminal prosecution.
"Speculation has been rife for many months now as to the status of the Stobie case, in so far as people were waiting for it to collapse," he said.
"It was a question of when and not if. My family's position has been completely vindicated."
He said the British Government could "not duck its responsibilities" and must implement a full independent judicial inquiry into the killing.
That call was echoed by Sinn Fein President Gerry Adams, who said the collapse of the trial was predictable.
"From the begnining the British system, and in particular the intelligence services responsible for collusion and the running of agents within the loyalist death squads, have used every means available to them to prevent the truth from emerging around Pat Finucane's case and hundreds of other similar killings," he said.
The collapse of the case also meant a not guilty verdict being returned on Mr Stobie over Mr Lambert's murder.
The Protestant student was shot dead by loyalist gunmen who mistook him for a Catholic when he was working on a north Belfast building site in 1987.
Central
The director of public prosecutions had to decide whether the central witness must give evidence.
Last week, the crown court was told that if Mr Mulholland gave evidence in the trial it could cause a relapse in his mental health.
Mr Mulholland interviewed Mr Stobie in the early 1990s.
After the hearing, Hugh Orde, who leads the day-to-day running of the Stevens Inquiry into the allegations of collusion in the murder of Mr Finucane, said the acquittal was a matter for the DPP and the court.
He said it was important that the inquiry team were seen to be thorough and independent in their investigations.
He said the team's report into the allegations of collusion would be completed and presented to the chief constable early next year.