High Court Judge Rohini Walgama found Sampath Pushpa Vidanapathirana and his three accomplices guilty of killing Khuram Sheikh and raping his girlfriend Victoria at a southern tourist resort on Christmas in 2011.
The murder followed an altercation between Vidanapathirana, the Chairman of the Tangalle Town Council, and 32-year-old Sheikh, an International Red Cross aid worker.
The trial raised much controversy when main accused Vidanapathirana, a member of President Mahinda Rajapaksa's Freedom Alliance, who was released on bail last year.
He was re-arrested and remanded in March this year until the end of the trial.
Britain's Prince Charles during his Sri Lanka visit in November raised the issue with President Rajapaksa. The case had been discussed in British Parliament.
British Prime Minister David Cameron had cited Sheikh's murder as an example of Lanka's failure to deliver justice.
Six people were indicted in December on the charges. The court discharged two of them today.