Born in Lund in the south of Sweden, Hillarp was the daughter of Nils Bengtsson and Hulda Johansson who ran a hardware business. At the age of 18, she married her former French teacher (until the marriage broke up in 1947). The couple moved to Stockholm where she studied literature and languages, graduating in 1945. While a student, she met the poet and editor Karl Vennberg who became a lifelong friend, but it was above all the work of Erik Lindegren which encouraged her to take a serious interest in poetry.
In 1946, Hillarp published her first work, the modernist collection of poems Solens bruun, followed by Dina händers ekon (1948), and Båge av väntan (1950), all evoking the difficulties of sexual relationships in male-dominated society. She went on to write prose works on the same theme: Blodförmörkelse (1951), Sindhia (1954), En eld är havet (1956), and Kustlinje (1963).
In addition to her lyrical works, Hillarp worked in film and photography. She also wrote a diary, excerpts of which were published in 2011 in connection with Birgitta Holm's biography, Rut Hillarp. Poet och erotissk geni (Poet and Erotic Genius). In particular, the biography provides accounts of her passionate relationships with the Romanian film director Mihail Livada, the author Sivar Arnér, and her colleagues and travel companions Gösta Oswald, Stig Sjödin and Bertil Schütt.