He was commissioned as a captain in the Australian Army Medical Corps in December 1916 and embarked for the Middle East on 17 February 1917, leaving his medical practice in Nowra, on the NSW south coast, in the care of a locum. He had purchased the practice in December 1915 and soon became the family doctor to the household living at Meroogal in West Street, Nowra: the elderly widow Jessie Thorburn and her unmarried daughters Belle, Georgie, Kate and Tottie. It was Cecil Molesworth who was called to certify Jessie Thorburn’s death at the age of 91 in May 1916.
Cecil spent all of his war service in the Middle East, most of the time as part of the Egyptian Expeditionary Force in Egypt and Palestine. He maintained a regular correspondence with the Meroogal women and other members of the extended Thorburn family, including their nieces Helen and Elgin Macgregor. In a letter written to Helen from Palestine in April 1918, Cecil reported that in one week he had had letters from Miss Tottie, Miss Kate and the ‘Brat’ (Elgin). From Helen he received a parcel of comforts, including passionfruit, which he relished as ‘one of the best reminders of home’.
In the final weeks of the war, Cecil was transferred to the 3rd Light Horse Field Ambulance. By early 1919 he was expecting to embark for Australia, telling Tottie Thorburn in a letter written from Zagazig, Egypt, on 21 March that he was ‘in a big hurry to get back’, to resume his practice in Nowra and to get married. He told Tottie that she would be the first in Nowra to know that he was engaged to a Sydney girl who was serving in Egypt as a nurse at the 14th Australian General Hospital. Cecil and his fiancée, Constance Kendall, planned to marry quietly in Sydney on their return, but this plan fell through when the 3rd Light Horse were deployed in the unhappy work of suppressing the Egyptian popular revolt that broke out in early March. Cecil and Constance married in Cairo in June 1919, sailed for Australia in July and, late in 1919, set up house in Nowra. Cecil became president of the Nowra Returned Soldiers’ League and a leading light in the campaign to establish a soldiers’ memorial hospital.