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."

Most flights coming out of or landing in Chile's main airports were cancelled or delayed.
The strike hit Chile in the busy run-up to the Christmas holiday period, leaving thousands of people stranded.
The airport ground workers' union called the strike after the government rejected their request to join the more generous Chilean military pension plan.
Some 3,000 luggage handlers, airport security and weather forecasters at 12 Chilean airports joined the strike.
Airport workers had gone on strike for 24 hours in September over the same issue.
This time the stoppage was meant to last 48 hours but it took four days for both sides to reach an understanding.
Their differences have not been solved, but the employees have agreed to resume talks.
Chilean Defence Minister Jose Antonio Gomez apologised for the disruption and said that airport staff would put in extra effort to try to bring the situation back to normal as quickly as possible.
"We will go the extra mile together with the airlines, with all of our staff and also with workers from the civil aviation authority to reverse this situation," said Mr Gomez.
Chile's flagship carrier, LAN, which is part of LATAM Airlines, said it would add 16 domestic flights to its daily schedule in the next few days.
But the airlines it will take days to take everyone to their destinations, as flights were already booked up before the strike began on Thursday.

"What time does your website close?" in Poole and "Can I exercise my kestrel on your tip?" in Nottingham are among the top ten bizarre calls of the last year.
Three of ten were made in Somerset, including: "What are the rules and regulations for hosting a mouse race?"
Councils are people's "first port of call" whatever the query, says the LGA.
The Local Government Association said councils' customer service hubs in England and Wales handled more than 50 million calls a year - the bulk of which are about council tax and parking.
But it compiled its top ten of the "slightly amusing to the outright bizarre" queries received in the past 12 months.
Peter Fleming, of the LGA, said: "These light-hearted examples of calls highlight the variety of issues that councils deal with every day."
A Sri Lankan court has sentenced a former MP to death for the murder of a rival politician five years ago.
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Airport workers in Chile have agreed to end a four-day strike that led to major disruption.
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Questions on everything from James Bond villains to Great British Bake Off cake tins have been received by council call centres in England.