Kate Mackay was born in a Victorian country town, Bendigo, the daughter of a presbyterian minister, one of five daughters. Her secondary education was at the Presbyterian Ladies' College, East Melbourne and from there she entered Melbourne University Medical School graduating in 1922. It was a distinguished year for women graduates, at least five went on to have excellent medical reputations in melbourne in specialised fields, they all joined the honorary staff of the Queen Victoria Hospital, Kate in 1927, and were responsible for the development of that hospital in the scientific aspect of medicine. Kate Mackay was the first consultant physician at that hospital and highly respected.

Her special interests were the growth areas of cardiology and endocrinology and in 1938 she went to Boston to study with the renowned Paul Dudley White. During World War II she served part-time as a major in the Australian Army Medical Corps and an additional duty was medical officer in the diabetic clinic of the Royal Melbourne Hospital, her old teaching hospital. She became very successful in both private and public practice and she had a long business connection as consultant physician for Myer, Melbourne, staff medical clinic.

She was also a cultivated woman, knowledgeable in the arts and able to appreciate contemporary painting, and her consulting rooms, as well as her home, were decorated with very striking pictures, an encouragement to young artists. Her friendships were wide, always the solid group of contemporary medical graduates, remarkably loyal to each other, but also artists and businessmen and the broad field of members of the Lyceum Club, and she was close to her sisters and their families, an affectionate and 'fun' aunt. All enjoyed her dry humour and astute comments and respected her professional ability and integrity.