Justice Lessit said Sergeant Fredrick Ole Leliman was the mastermind of the "sophisticated murder".
The High Court in Kenya has sentenced former police officer Sergeant Fredrick Ole Leliman to death for the murder of human rights lawyer Willie Kimani and two others in 2016.
Kenya has not executed anyone since 1987, with death sentences usually being commuted to life in prison.
Justice Jesie Lessit said Leliman was the mastermind of the "sophisticated murder". She further said that the murder was most foul, with meticulous planning and execution.
Police officers Stephen Cheburet and Sylvia Wanjiku were sentenced to 30 and 24 years in jail respectively while police informer Peter Ngugi was jailed for 20 years.
Justice Jessie Lessit, now a judge of the Court of Appeal, found them guilty of killing the rights lawyer, his client Josephat Mwenda and taxi driver Joseph Muiruri on June 23, 2016.
The bodies of the three men were found stashed in gunny bags in Ol Donyo Sabuk, Machakos County on July 1, 2016.
They had been dumped in River Athi and were found more than a week after the men were reported missing.
At the time of his death, lawyer Kimani was working for International Justice Mission which helps investigate and document police killings and brutality.