He became Chief Constable in 1980, after an attachment to Scotland Yard. As Chief Constable, he changed the interview processes of terrorist suspects at the Castlereagh interrogation centre. An anonymous former interrogator has claimed that "The new chief constable was completely against any mistreatment of prisoners whatsoever...we started to detect a change ... straight away." Hermon is thought to have believed that the allegations of mistreatment were harming relations between the RUC and the wider communities. He retired in 1989, and became the longest-serving Chief Constable of the RUC.