The Judge collapsed at Parklands Sports Club on Thursday evening and was later on rushed to MP Shah Hospital where he was pronounced dead before arrival.
Justice Onguto is said to have finished a gym work out session at the club and had just sat down to have a refreshment before the tragedy occurred.
Police sources say autopsy reports will assist determine what could have killed the high court judge.
Chief Justice David Maraga in his condolences to the family termed Onguto as a dedicated judge and a team leader, who brought into the judiciary extensive experience as a legal practitioner.
Onguto was appointed in July 2014 by President Uhuru Kenyatta appointed Onguto in July 2014 and was among the 11 lawyers who had been nominated by the Judicial Service Commission (JSC).
He served in the Environment and Land Court Division until 2015 when he was transferred to the Constitutional and Human Rights Division.