Iestyn Davies, 54, was convicted by a 10 to 2 majority verdict after he denied killing the 22-year-old care worker.
The judge at Caernarfon Crown Court described Davies as "very dangerous" and "calculating and devious".
Ms Roberts' body was found in a drainage ditch on 10 April last year.
The jury deliberated for just over 21 hours before reaching a verdict.
Sentencing Davies, Mr Justice Lloyd-Jones said: "I'm sure this murder has a sexual motive which must have increased Ffion's ordeal.
"You have been found guilty on the clearest and the most compelling evidence.
"This is a foul crime which involved the wicked taking of a young life."The trial heard that the case against Davies was that he was in Porthmadog town centre early on the morning Ms Roberts was killed.
He was seen on CCTV around 0400 BST, but disappeared from view shortly before Ms Roberts was last seen.
Items of clothing from the dead woman were then linked to him by DNA evidence.
The court heard that tracksuit bottoms, a training shoe and Ms Roberts's handbag were found because they got lodged in the mechanism of sluice gates not far from where her body was found.
Describing Ffion as easy prey because she was drunk, the judge added: "It was a terrible mischance that your paths crossed, on an impulse you saw your chance and you took it."Iestyn Davies must serve a minimum of 25 years before parole
Police visited 1,340 homes and spoke to 4,020 of the town's occupants during the investigation.
As part of their inquiries, Ms Roberts's brother Elgan and father Idris were arrested but later released without charge.
In a statement, Ffion's family thanked everyone who had been so supportive to them and those who had helped bring her killer to justice.
"Nothing can take away the pain of this terrible crime," they said.
"The family is pleased that justice has now been done and they request that they be given some privacy to rebuild their lives and to mourn the loss of Ffion."
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