Theresa Whistler was born Theresa Furse in London in 1927 to civil servant Sir Ralph Furse, and was the grand daughter of poet and Sir Henry Newbolt. She therefore grew up rooted in literature and literary figures, through this becoming close friends with Walter de la Mare.

After she was awarded a first class degree in English Literature at Oxford University, Theresa published The Collected Poems of Mary Coleridge and a childrens book, The River boy in the mid 1950s. In the late 1950s she began working on what would eventually be the biographical work Imagination of the Heart: The life of Walter de la Mare, a project that due to her domestic duties of bringing up her late elder sister's children would take nearly thirty five years.