Duminda Silva was found guilty with four others of shooting dead Bharatha Lakshman Premachandra and three of his supporters during local elections in Colombo in 2011.
The two men were both members of the then governing party.
The defendants are likely to serve long jail terms because of Sri Lanka's moratorium on executions.
Their lawyers said they would appeal against the sentences.
Silva sustained gunshot wounds and was flown out of the country, apparently for treatment, after the shooting.
He was a close ally of the powerful Defence Secretary Gotabaya Rajapaksa, whose brother Mahinda was president until losing elections in 2015.
Many hailed the verdict as proof that judicial independence had been restored in Sri Lanka.
"This verdict was only possible because we have a new president," Hirunika Premachandra, the murdered man's daughter who is now an MP herself, told reporters.
"I hope there will be justice for all the murders that happened during the previous era, including the killing of former editor Lasantha Wickrematunge, disappeared journalist Prageeth Ekneligoda and rugby player Wasim Thajudeen, so their families can get justice."
A Sri Lankan court has sentenced a former MP to death for the murder of a rival politician five years ago.