A jury unanimously convicted 43-year-old Ali Qazimaj of the murders of Peter Stuart, 75, and his wife Sylvia Stuart, 69.
Mr Stuart was found stabbed in shallow water near his home in Weybread, Suffolk, on June 3 last year. Mrs Sylvia is still missing but she is presumed dead.
Qazimaj had claimed he had never been to the UK before his arrest and extradition from Luxembourg last June. He had denied murder.
But Karim Khalil QC, prosecuting, said he had lived in Tilbury, Essex, and worked as a carer to the father-in-law of the Stuarts’ daughter, Christy Paxman.
He added Qazimaj had amassed gambling debts and thought the Stuarts were millionaires.
Detective Chief Inspector Andy Guy, of Norfolk and Suffolk’s Major Investigation Team, said: ‘There were four key strands of evidence – the fingerprints, the DNA, the handwriting samples and the physical appearance.
In a statement read outside court, Mr and Mrs Stuart’s family added: ‘To the twisted individual who committed this wicked crime: we hope you spend the rest of your miserable existence reflecting on the utter senselessness and brutality of what you did to two innocent people, and that maybe one day you will find the moral courage to tell us where mum is so that we may give her and our family some final peace.’
The judge at Ipswich Crown Court told Qazimaj he could expect a mandatory life prison term.