Brentnall studied medicine at the University of Manchester, qualifying in 1915. During the First World War, he served with the RAMC, rising to the rank of major and being awarded the MC. He worked at the Liverpool Northern Hospital, before joining St. Mary's in Manchester, where he was elected to the honorary staff. He also held appointments at Stretford Memorial Hospital and the Warrington Infirmary. Brentnall was a gynaecologist, and was a founder member of the College of Gynaecologists and Obstetricians. He published a number of papers with the North of England Obstetrical and Gynaecological Society. He was elected president of this Society in 1946. He was the brother of Edward Brentnall.