Nicky Jacobs was driven from Belmarsh prison, in south-east London, in a silver saloon after administrative delays that delayed his freedom yesterday were completed.
Speaking outside the prison, his lawyer, Tony Meisels, said his client was "relieved" to be free after a four-year ordeal.
He said police and prosecutors must investigate why the case was brought, as the jury took just hours to acquit him after a trial lasting weeks.
He said: "Our thoughts are with PC Blakelock's widow and her children. Obviously, they have not seen justice and have been dragged through the process as much as Mr Jacobs has.
"Questions need to be asked at the very highest levels of the Crown Prosecution Service and the police as to why this prosecution was ever brought."
Jacobs was found not guilty of the murder of PC Blakelock by a jury at the Old Bailey in London yesterday.
The officer was attacked by an armed mob after his unit was deployed to protect firefighters tackling a blaze on the Broadwater Farm estate in north London on the night of October 6, 1985.
Jacobs, who was 16 at the time of the riots, was the seventh person to be charged with the officer's murder, but no-one has been brought to justice for the killing.
The Crown Prosecution Service said yesterday that while it accepted the verdict, "it was right" to bring the case against Jacobs.
Scotland Yard Assistant Commissioner Mark Rowley vowed: "We will not give up on bringing Keith's killers to justice."
He said: "Sadly, Keith's widow, family and friends still have not seen anyone brought to justice for his murder."
The dignity, extraordinary patience and courage they have shown in their nearly 30-year quest for justice is humbling."
Jacobs's prosecution followed an earlier trial in 1987, when three men were convicted of the murder but later freed on appeal.
The case against three youths was dropped.