She received her
bachelor's degree and Board of Education Certificate in Teaching in 1921 from
Reading University, and then worked as a teacher before being employed by the
King's College of Household and Social Science as a part-time demonstrator, spending most of her time on research. She was taught by
Dr. Nellie B. Eales when associated with the
Freshwater Biological Association. She met
Charles Regan and was employed by him as his assistant until hired by the British Museum (Natural History) as Assistant Keeper in 1935. She served as the senior scientist in the Fish Section of the British Museum (Natural History) for almost 50 years, and was known internationally as an authority on several diverse groups of fishes. She was best known for her work describing
African
Rift lake cichlids, but she published extensively on other groups as well. She used laboratory study and extended field trips to research her current areas of study, and often relied on interviews with local people to understand the behaviours, forms and food potential of fishes. One example of the influence of Regan and Trewavas is that of the currently named
genus categories of
Lake Malawi mbuna, two are attributed to Regan and six to Trewavas. Of the
Haplochromis sensu lato in the lake, five were described by Regan and twenty-seven by Trewavas, either individually or in partnership with David Eccles. Later in life she mentored prominent researcher
Ad Konings, who has continued many of her areas of study. When her eyesight failed she insisted that he accept her stereo microscope as a gift so he could continue her work with African cichlids. Trewavas died in Reading on 16 August 1993. ==Honours==