Born in Galaţi, she was married with fellow writer Vladimir Colin in 1943 (divorced in 1948), and later with Al. I. Ştefănescu. At the beginning of her career, Cassian was, with Colin, one of the noted contributors to the magazine Orizont. She also had a very close relation with Ion Barbu, one of the most important Romanian poets and mathematicians.
Cassian travelled to the United States as a visiting professor in 1985. During her stay in America, a friend of hers, Gheorghe Ursu, was arrested by the Securitate for possessing a diary. The diary contained several of Cassian's poems which satirized the Communist regime and the authorities thought to be inflammatory. Hence, she decided to remain in the US. She was granted asylum in the United States, and she currently resides in New York City.
Her poetry currently remains somewhat controversial (more appreciated by readers than by critics, who often pointed out her compromises with the Communist regime). Her debut volume, La scara 1/1 (1947), was demolished by socialist realist critics and banned (later though, some of the poems from the book had become famous for lines such as the following: "M-am deplasat la fața locului/și fata locului era umflată”, Tumefiere); as a consequence, Cassian disavowed her own volume and adopts socialist realism (in volumes such as An viu - nouă sute şi şaptesprezece) in order to be published. On one hand, she enjoyed the priviledges of writing according to the Communist dogma (and, actually, did not ever reject her leftist views); on the other hand, while sporadically contributing to official poetry anthologies until the 1980's, she took advantage of every opportunity to escape the box of socialist realism, including the advent of children's literature (in the first half of the 1950's) and the return of lyrical themes after 1956 (partially thanks to Nicolae Labiș). Some of the poems of this period predate the publication of Ana Blandiana's neomodernist volumes.
In the second half of the 1960's, when literature was now almost free of limits, Nina Cassian reissues La scara 1/1 (which, reread now, seems to partially anticipate oneirism and mannerism, but was actually elaborated from influences of Arghezi, Barbu and surrealism) and continues her initial style with volumes that are possibly influenced by (then recently arrived) oneirism and neo-expressionism. One of her most popular volumes of the period between 1965 and the early 1980's is the mannerist-tinted Loto-Poeme (1972), which was singled out by critic Ion Pop for its ingenious experimental poems written in a imaginary dialect (limba spargă). Virgil Teodorescu hinted in one of his poems that Cassian's imaginary language was inspired by his 1940 experiment from Poem în leopardă (even though there are structural differences between the two invented languages).
Since the beginning of her exile, she has published several books in English, including Take My Word for It, Cheerleader for a Funeral, Life Sentences, Continuum etc. The multi-volume autobiographical book Memoria ca zestre was published in Romania and enjoyed success, as well as being the subject of controversies related to her numerous love affairs and her low reputation (due to having been one of the central figures of socialist realism).