The convicted killer and paedophile was found guilty of raping and murdering teenage girls Christine Eadie and Helen Scott 37 years ago.
The pair, both 17, were brutally killed after a night out at Edinburgh’s World’s End pub on October 15 1977, with their bodies discovered the following day in East Lothian. They had been bound and strangled with their own underwear.
Sinclair, 69, was convicted today of the rape and murder of both girls after a trial at the High Court in Livingston lasting five weeks.
He was jailed for life and ordered to spend at least 37 years behind bars.
Judge Lord Matthews told him he was “a dangerous predator who is capable of sinking to the depths of depravity”.
The prosecution was the first under changes to Scotland’s double jeopardy law which meant he could be retried for their murders after the court case against him collapsed seven years ago.
Sinclair, a serial rapist who has been in jail for more than 30 years, was accused of carrying out the attacks with his brother-in-law Gordon Hamilton, who died in 1996.
The jury of nine women and six men took less than two-and-a-half hours to convict Sinclair unanimously of both charges.
In reaching their verdict, jurors were unaware that Sinclair has already spent more than half of his life in prison.
He was just 16 when he strangled seven-year-old Catherine Reehill in Glasgow in 1961, later pleading guilty to a charge of culpable homicide and serving six years.
In 1982 he was convicted at the High Court in Edinburgh of a string of sex attacks on 11 young girls - including three rapes.
While still in prison, he was given a life sentence in 2001 for the murder of 17-year-old Mary Gallacher, who was raped and stabbed in Glasgow in 1978.
The case, which became known as the World’s End murders, was for decades one of Scotland’s highest-profile unsolved crimes.
The discovery of the girls’ bodies on October 16 1977, dumped in remote locations around five miles apart from each other, conveyed the unimaginable horror they suffered at the end of their all-too-short lives.
Christine’s naked body was found at around 2.25pm that day at Gosford Bay, Aberlady. She had a ligature around her neck, her mouth was gagged with a pair of pants and her wrists had been tied behind her back.
She had been punched and kicked on her head and body, bitten, raped, bound and strangled.
Helen’s partially-clothed body, discovered at around 6pm, had been dumped in a wheat field at the Huntington to Coates road, near Haddington.
Her hands were also tied behind her back and a ligature made from a belt and a pair of tights had been used around her neck.
Raped, bound and throttled, she too had been punched and kicked, and her head had been stamped on.
She might have been forced to walk barefoot into the field where she was found dead.
Days into the trial, the jury of nine women and six men would return to the isolated spots where the girls’ bodies had been left, 37 years on from the brutal killings.
The court was told about the “terrible and life-changing” effect the murder had on Helen’s family.
Her father told the court his wife Margaret, who died in 1989, was never the same again after their daughter’s death.
Morain Scott, 84, said the death of his daughter marked the start of his wife’s ill health, while he has lived with the loss for more than 30 years and has “just kept going”.
In the words of the prosecutor Lord Advocate Frank Mulholland, as the girls begged to be freed, Sinclair and Hamilton ended their lives “like something that was wiped off a shoe”.
Sentencing Sinclair, Lord Matthews said that whatever dreams the girls had for their futures had “turned to nightmares” that night when they left the World’s End pub, “the name of which has become synonymous with these notorious murders”.
He said: “Little were they to know that they had the misfortune to be in the company of two men for whom the words evil and monster seem inadequate.
“Unless one day your conscience, if you have one, motivates you to tell the truth, no one other than you will ever know precisely what part you and Gordon Hamilton played in these awful events.
“Perhaps it does not matter. What does matter is that the girls were subjected to an ordeal beyond comprehension and then left like carrion, exposed for all to see, with no dignity, even in death.
“For them at least the nightmare is over and if they were not resting in peace before today I hope that they are now.
“The nightmare for their families and friends, on the other hand, has gone on from those first awful moments when they heard the news no one should hear until even now, 37 years later and counting. It will never end.
“No one who saw the evidence of Helen’s father, sisters and boyfriend and Christine’s mother could fail to have been moved by it. They are an example to us all, waiting patiently for justice while the authorities have worked tirelessly to achieve it.”
Lord Matthews said Sinclair had shown “not one ounce of remorse” for his “terrible deeds”.
He said: “The evidence in this case as well as your record, details of which have now been revealed, shows that you are a dangerous predator, who is capable of sinking to the depths of depravity.
“I do not intend to waste many words on you. You are well aware that the only sentence I can pass is one of life imprisonment.”
The judge also thanked the jury for the attention they had paid to the case and for “participating in legal history”.
He said: “I don’t think any of you will easily forget the photographs which we saw, showing the terrible contrast between two young girls with everything to live for and their two corpses left to rot in East Lothian.
“At this time of year and particularly on Monday we heard the famous words of Laurence Binyon in his poem For the Fallen. That was written for the young people who gave their lives in the Great War but may have some resonance for the families of Christine and Helen:
‘They shall grow not old as we that are left grow old.
‘Age shall not weary them nor the years condemn.
‘At the going down of the sun and in the morning we will remember them.’
“I say that because, while all of their loved ones would have wished to see them live on to a ripe old age, the memories they will have of them will always be of two happy home-loving innocent girls unbesmirched by the ravages of time.
“That is, indeed, how I think the whole of the country will remember them.”