Following graduation Douglas worked for ten years as a costume designer in Montreal and New York.[1][2] A career change landed her at McGill University, where she worked as a lecturer in English (1947–1959) and as a researcher on a project related to what would become the Burney Centre.[2] She later worked as an archivist from 1978 to 1981 at the Dr. Wilder Penfield Collection at the Montreal Neurological Institute.[2][1] During this period she began working as a genealogist and in 1989 was certified by the Genealogical Institute of the Maritimes.[2]
In 1982 Douglas and her husband relocated to Toronto, where both worked for IMAX.[2][1] Douglas began as a part-time employee editing standard manuals, and eventually becoming an author of specialised manuals for use of IMAX technology in various countries based on the region's technological context.[1] As an employee at IMAX, she was an early adopter of AutoCAD to facilitate the use of diagrams, and subsequent updates, as part of her technical work.[4][1] Outside of work, Douglas continued archival and genealogical work, establishing "Althea Douglas Consultants".[1] One of her projects consisted of working for the Girl Guides of Canada to establish the arrangement structure for their archives, as well as developing a catalogue and digital finding aids.[1]
Following a move to Ottawa, Douglas transitioned to work as a professional genealogist researching and publishing a number of works through the Ontario Genealogical Society. In addition to her own books, Canadian Railway Records – A Guide for Genealogists (1994) was co-authored with her husband. In a review of "Help! I’ve Inherited an Attic Full of History", Douglas' guide for non-archivists, Candice Vetter called the work "useful and easy to follow" containing "advice essential for the layperson".[5]
