Seven others were acquitted during the trial which began in 2012. The accused were charged of murdering Bharatha Laxman Premachandra in October 2011 ahead of the elections.
Duminda Silva and Premachandra had exchanged blows and Premachandra was subsequently gunned down.
Rajapaksa government was accused of covering up and protecting Silva who was closed to him and his brother Gotabhaya Rajapaksa, a top civil servant in the defence ministry at that time.
Premachandra's daughter Hirunika seeking justice for her father's murder backed the current President Sirisena against Rajapaksa. She is now a parliamentarian in the current unity government.
A 3 member bench was divided 2-1 on the ruling. The death penalty is commuted to a life term imprisonment in Sri Lanka where none of the condemned were hanged since May of 1976.