Menhennitt was appointed as an Acting Judge of the Supreme Court of Victoria on 27 April 1966, and quickly impressed with his capabilities. The "Australian Bar Gazette" stated that during his time in the position, he had "revealed learning and skill in the conduct of trials in matters far removed from his previous fields of practice." Six months later, Sir Norman O'Bryan retired from the bench, and on 17 October, Menhennitt was appointed as his replacement. Menhennitt was highly regarded for both his knowledge and efficiency during his time on the court, and had a reputation for being relatively quick in returning decisions. While on the court, he was the original judge in charge of the Building Cases List, and was later responsible for the Commercial Causes List. He also served as chairman of Victorian Council of Law Reporting from 1974 until his death, and had also been a member of the Chief Justice's Law Reform Committee.