In 1934, he began studying at the Society of Oxford Home Students at the University of Oxford and became captain of the women’s rowing team. After graduation, he started working in a laboratory near Bristol. Around this time, Dillon became aware of a doctor who had been studying the effects of testosterone on female patients, and started taking the hormone for personal use, driven by a desire to become a man. Dillon left his job at the laboratory after he was outed to his colleagues. He subsequently found a job as a petrol pump attendant in a garage in Bristol and worked there during World War II. Whilst at the garage, he began writing what would become his 1946 book Self: A Study in Ethics and Endocrinology, considered to be a pioneering work in the field of transgender medicine. He also received a gender-affirming double mastectomy whilst in hospital for hypoglycemia and heard of the work of surgeon Sir Harold Gillies, who agreed to perform a phalloplasty on Dillon after the war.

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