Cyril Tewsley was born in Dunedin, graduated MA at Otago University, and, like many New Zealanders of his time, gained his medical education in Edinburgh. Returning to New Zealand in 1909, he practised in Sandringham. He served in the NZMC in the First World War and was at the Gallipoli landing. Later he was officer-in-charge of the Convalescent Hospital in Hornchurch. After the War he began as a consultant physician in O'Rourke Street. For many years he was honorary senior physician at Auckland Hospital. He had a reserved and brusque personality but under this was sensitive and kind. He was kind and helpful to his juniors and supportive to his seniors.

He received the CMG for his work in the First World War. In the Second World War he was ADMS Northern Command with the rank of colonel. From 1948 to 1950 he was New Zealand vice-president of The Royal Australasian College of Physicians. Earlier he had served as chairman of the local division of the BMA.

Dr Tewsley was a Commander in the Order of St John, chief medical officer of the St John Ambulance Association and a member of the Auckland centre committee. He was an office-bearer of the Cathedral Church of St Mary. He was a keen swimmer and tennis player and had great interest in botany and horticulture. His ward rounds were punctual, precise and stern. He was not accustomed to levity, and medical students felt discouraged and tended to miss his ward rounds. More senior people gained much more benefit. In his own home Dr Tewsley and his wife entertained with grace and charm. Here the atmosphere was ever one of hospitality and goodwill. There were five children of the marriage, two sons and three daughters, two of whom married and live in England.