He was admitted to legal practice in 1935 with solicitor firm Alexander Grant, Dickson and Menhennitt, and worked there until his admission to the bar in 1946.
A talented litigator, he appeared many times before the High Court of Australia and Privy Council, and was appointed as a Queen's Counsel in 1957.
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."
Menhennitt died suddenly at his North Balwyn home on 29 October 1979, the day before his 67th birthday, and was found by his driver, who had come to take him to the court.
The Supreme Court held a special sitting in his honour the following day, whereupon Chief Justice Sir John Young paid tribute to Menhennitt's distinguished career.