Andrew Joseph Brenan was the youngest son of James and Annie Brenan. James Brenan, a Melbourne City councillor for twenty years, had served as a board member at the Royal Melbourne Hospital. His wife Annie was a board member at the Royal Women's and Fairfield Infectious Diseases Hospitals. Following matriculation at Melbourne University in April 1904, Andrew Brenan completed his medical course in 1909. He was appointed resident medical officer at St Vincent's Hospital in that year and in the following year was appointed registrar, clinical pathologist and assistant outpatient surgeon. In 1912 he was appointed pathologist and clinical pathologist and in 1913 was appointed an outpatient surgeon. Following his appointment as a pathologist, he worked closely with Sir Thomas Dunhill who, at that time, was a world leader in thyroid surgery. Andrew Brenan obtained the degrees MD (Melbourne) in June 1911 and MS (Melbourne) in February 1915.

Andrew Brenan left Australia on 17 May 1915 with the 3rd AGH and served with this unit in the Middle East and at Lemnos during the Gallipoli campaign. He later served in France with the 1st AGH and with the 3rd Field Ambulance. Following the armistice he worked in a military hospital in England and returned to Australia in May 1919. He was an honorary lieutenant colonel, AAMC until December 1937. When on military service in France he had worked with a number of leading French pathologists. He returned to visit these colleagues in France and also spent time in Vienna and at the Rockefeller Institute in New York in 1926. In 1919 following his return to Australia he resigned his position as outpatient surgeon and was appointed director of pathology at St Vincent's Hospital. He retired as director in 1948 and then continued as clinical pathologist and director of laboratories until he completed his hospital service in 1959.

Andrew Brenan was a superb teacher and was responsible for setting up a pathological museum which enabled both graduate and postgraduate students, not only to learn, but to develop an interest in clinical pathology. He was appointed as an honorary lecturer in the pathology department at the Melbourne University in 1925 and was an examiner in pathology for the MD degree at Melbourne University for thirty years. Andrew Brenan encouraged clinicians to attend the autopsy room to improve their clinical skills. His aim was not to embarrass, he believed that a knowledge of pathology was essential for good clinical practice. Consultant physicians at the Hospital were encouraged to participate in post mortem examinations.

For many years he was a member of the advisory council at St Vincent's Hospital. In this capacity he showed great loyalty to the Sisters of Charity. He was a man of vision and was responsible for a number of initiatives which resulted in extremely high standards at the Hospital. He was a superb chairman of committees and impatient with indecision. Andrew Brenan played an important role in setting up the special clinics at the Hospital. The orthopaedic, urology and neurosurgical clinics developed a high standard and won world wide recognition. He was well known for his encouragement of postgraduate students and emphasised the value of overseas training. Besides his hospital work Andrew Brenan worked as a consultant pathologist and was highly regarded by consultants in all Melbourne teaching hospitals. His opinion on the histology of surgical specimens was especially valued. When the royal colleges were formed, Andrew Brenan became – because of his qualifications, experience and reputation – a foundation fellow of the Royal Australasian College of Physicians and the Royal Australasian College of Surgeons. It is of interest that he was originally a member of the Association of Physicians of Australia. In 1956 he was appointed as a member of the College of Pathologists of Australia.

Andrew Brenan was a member of a well known family with a long interest in ballet and the theatre. His sisters Jennie and Eileen were regular hostesses to visiting theatrical celebrities for over fifty years. He had a special interest in horse racing and attended metropolitan race meetings regularly for over seventy years. He was a life member of the Victoria Racing Club. Andrew Brenan, one 'of a group of men who have combined intellectual prominence with nobility of character' had a profound influence on the medical scene in Melbourne for over forty years. He was extraordinarily popular; it felt so much better to have known him. He married Muriel Bridge in 1926 and had two children, John Andrew Brenan FRACP, consultant dermatologist, and Anne Heath, a dental surgeon.