After graduation Ashby held house appointments at the Derbyshire Royal Infirmary and the Norfolk and Norwich Hospital. He later entered general practice at Aylesbury. He served in the Royal Army Medical Corps during the First World War and was present at the Battle of the Somme, and also served in Egypt and Palestine. Following the war he returned to his general practice in Aylesbury, and was later appointed surgeon-in-ordinary to the Royal Buckinghamshire Hospital. Outside of medicine, he was elected to the Buckinghamshire County Council. He later moved to Cardiff, retiring to Cork in 1953 having suffered an oedema.