In 1891, Thomas decided to set up a business in organ-building and the family moved to London. There, Casson was educated at St Mary's College, Paddington before going to secretarial college at her father's behest. After qualifying, she worked as a secretary for her fathers' organ-building business until his retirement. In 1908, Casson was a working as housing manager for the social reformer, Octavia Hill, taking care of the living conditions of residents at Red Cross Hall in Southwark. For the next five years she organised recreational activities at the hall, focusing on the arts. Octavia Hill's philosophy influenced Casson's work over the following years.