Casson became a medical officer at the Holloway Sanatorium in 1921, developing an interest in occupational therapy whilst working there until 1929. In that time she gained a diploma in psychological medicine from the University of London in 1922, achieved her Doctorate of Medicine from the University of Bristol in 1926 (becoming the first woman to do so), and winning the 1927 Gaskell prize from the Royal Medico-Psychological Association. She also helped set up the Standing Committee in Psychological Medicine of the Medical Women's Federation and the Gaskell club for those recipients of the prize.