Rubens was born in Cardiff, Wales in 1928. Her father, Eli Rubens, was a Lithuanian Jew who, at the age of 16, left mainland Europe in 1900 in the hope of starting a new life in New York. Due to being swindled by a ticket tour, Rubens never reached America, his passage taking him no further than Cardiff. He decided to stay in Wales, and there he met and married Dorothy Cohen, whose Polish family had also emigrated to Cardiff. Rubens was one of four children and came from a musical family, both her brothers, Harold and Cyril, becoming well-known classical violinists. Harold was forced to quit playing through illness, but Cyril became a violinist in the London Symphony Orchestra. Rubens failed to follow in her family's musical tradition though she would later learn the piano and cello. She was educated at Cardiff High School for Girls and later read English at the University of Wales, Cardiff, where she was awarded her BA in 1947. She married Rudi Nassauer, a wine merchant and novelist. They had two daughters, Rebecca and Sharon.
From 1950 to 1955, Rubens taught at a grammar school in Birmingham, before moving onto the film industry where she made documentaries. In the 1960s they owned 10 Compayne Gardens, NW3, where the poet Jon Silkin rented the attic storey and sublet rooms to David Mercer, later a prolific West End and TV playwright, and Malcolm Ross-Macdonald, later an equally prolific writer of historical novels.