The four defendants were convicted of murder in July, 2022.
The lead defendant, Frederick Leliman, was sentenced to death by the court. However, Kenya has not executed anyone since 1987, with death sentences usually commuted to life in prison.
The other two police officers, Stephen Cheburet and Sylvia Wanjiku, and their civilian informant, Peter Ngugi, were given prison sentences ranging from 20 to 30 years.
Kimani, his client Josephat Mwendwa and their driver Joseph Muiruri were killed shortly after filing a complaint of police brutality alleging that Mwendwa had been shot and injured by police.
Their bodies were later recovered from a river outside the capital Nairobi.
The judge, Jessie Lessit, described the killing as a "well planned and executed" murder.
A police spokesperson said the police would issue a statement later.
The police say they take action against any officer accused of brutality, while the Independent Policing Oversight Authority, a body set up to probe cases of police brutality, investigates such cases and recommends them for prosecution.
At the time of his death, Kimani was working for International Justice Mission, a global legal rights group that helps investigate and document police killings and brutality.
The case triggered outrage in Kenya, where police face frequent allegations of brutality and extrajudicial killings but are almost never charged.