[1] He matriculated at the Gert Maritz School in Pietermaritzburg and attended the University of Natal, where he completed a BCom in 1980 and an LLB in 1982.
[1] He was chairperson of the Pietermaritzburg Society of Advocates in 2005, following a two-year term as vice-chairperson, and, on several occasions in 2004 and 2005, he was an acting judge in the KwaZulu-Natal Division of the High Court of South Africa.
[5] He handed down his first major ruling in that matter in October 2021, rejecting Zuma's special plea to halt prosecution on the grounds that, among other things, prosecutor Billy Downer was not competent to try the case.
[6][7] He denied Zuma leave to appeal the special plea judgement in February 2022,[8][9] as did the Constitutional Court in December 2022.
[12][13] When the trial resumed in January 2023, Koen recused himself, saying that "it is what the sound administration of justice, the constitution and my conscience dictate" because his prior statements on the alleged leak could create the perception of bias.
In February 2021, Koen was among eleven candidates whom the Judicial Service Commission shortlisted for possible appointment to one of five permanent vacancies at the Supreme Court of Appeal.
[25][26] He also told the interview panel that he had "felt gutted by the criticism that had been expressed of me" during his last Judicial Service Commission, and he also provided transcripts and audio recordings of the 2016 meeting which Mogoeng had described.
[27] Deputy Chief Justice Mandisa Maya said that Mogoeng's description did not fit with her perception of Koen, who, when acting at the appellate court, had "always behaved impeccably".