Born into a Jewish family in Galaţi (her father being Iosif Cassian-Mătăsaru, a translator), they lived in Brașov between 1926 and 1935. In 1935, the family moved to Bucharest, where she went to high school. She took drawing lessons with George Loewendal and M. H. Maxy, acting lessons with Beate Fredanov and Alexandru Finți, piano and musical composition lessons with Theodor Fuchs, Paul Jelescu, Mihail Jora and Constantin Silvestri.
She published her first poem, Am fost un poet decadent ("I Used to Be a Decadent Poet") in the daily România liberă in 1945, and her first poetry collection, La scara 1/1 (""Scale 1:1") in 1947. It was labeled "decadent poetry" in a Scînteia article in 1948. Scared by that fierce criticism, she then turned to writing in the proletkult and socialist-realistic fashion. This phase lasted for about eight years.
At the beginning of her career, Cassian and her first husband, Vladimir Colin, were contributors to the magazine "Orizont".
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 and subsequently beaten to death 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.
- La scara 1/1, Bucharest, 1947
- Sufletul nostru, Bucharest, 1949
- An viu nouă sute şaptesprezece, Bucharest, 1949
- Nică fără frică, Bucharest, 1950
- Ce-a văzut Oana, Bucharest, 1952
- Horea nu mai este singur, Bucharest, 1952
- Tinereţe, Bucharest, 1953
- Florile patriei, Bucharest, 1954
- Versuri alese, Bucharest, 1955
- Vârstele anului, Bucharest, 1957
- Dialogul vântului cu marea, Bucharest, 1957
- Botgros, căţel fricos, Bucharest, 1957
- Prinţul Miorlau, Bucharest, 1957
- Chipuri hazlii pentru copii, Bucharest, 1958
- Aventurile lui Trompişor, Bucharest, 1959
- Încurcă-lume, Bucharest, 1961
- Sărbătorile zilnice, Bucharest, 1961
- Spectacol în aer liber. O monografie a dragostei, Bucharest, 1961
- Curcubeu, Bucharest, 1962
- Poezii, foreword by Ovid S. Crohmălniceanu, Bucharest, 1962
- Să ne facem daruri, Bucharest, 1963
- Disciplina harfei, Bucharest, 1965
- Îl cunoaşteţi pe Tică?, Bucharest, 1966
- Sângele, Bucharest, 1966
- Destinele paralele. La scara 1/1,1967
- Uite-l este... Uite-l nu e, Bucharest, 1967
- Ambitus, Bucharest, 1969
- Întâmplări cu haz, Bucharest, 1969
- Povestea a doi pui de tigru numiţi Ninigra şi Aligru, Bucharest, 1969
- Cronofagie. 1944-1969, Bucharest, 1970
- Recviem, Bucharest, 1971
- Marea conjugare, Bucharest, 1971
- Atât de grozavă şi adio. Confidenţe fictive, Bucharest, 1971; Second edition (Confidenţe fictive. Atât de grozavă şi adio şi alte proze), Bucharest, 1976
- Loto-Poeme, Bucharest, 1971
- Spectacol în aer liber. O (altă) monografie a dragostei, foreword by Liviu Călin, Bucharest, 1974
- Între noi, copii, Bucharest, 1974
- O sută de poeme, Bucharest, 1975
- Viraje-Virages, bilingual edition, translated by the author, Eugene Guillevic and Lily Denis, Bucharest, 1978
- De îndurare, Bucharest, 1981
- Blue Apple, translation by Eva Feiler, New York, 1981
- Numărătoarea inversă, Bucharest, 1983
- Jocuri de vacanţă, Bucharest, 1983
- Roşcată ca arama şi cei şapte şoricei, Bucharest, 1985
- Lady of Miracles, translation by Laura Schiff, Berkeley, 1988
- Call Yourself Alive, translation by Brenda Walker and Andreea Deletant, London, 1988
- Life Sentence, New York-London, 1990
- Cheerleader for a Funeral, translation by the author and Brenda Walker, London-Boston, 1992
- Desfacerea lumii, Bucharest, 1997
- Take My Word for It, New York, 1997
- Memoria ca zestre. Cartea I (1948-1953, 1975-1979, 1987-2003), Bucharest, 2003
She married Vladimir Colin in 1943 (divorced in 1948), and later Al. I. Ştefănescu. In the US, she was married to Maurice Edwards. Although born into a Jewish family, her second husband was Romanian Orthodox, and during their marriage, she stated that she was much closer to his religion than to Judaism, and that she had never read a page of the Talmud.
- Ștefănescu, Alex. "La o noua lectura: Nina Cassian" (in Romanian). Retrieved 16 April 2014.
- (7 March 1999). Poetry in Brief, The Independent
- "A murit poeta Nina Cassian. Cenușa sa va fi adusă în România" (in Romanian). Retrieved 17 April 2014.
- Ciuta, Larisa (16 April 2014). "A MURIT NINA CASSIAN. Povestea scriitoarei de care s-a indragostit Ion Barbu". Evenimentul zilei. Retrieved 16 April 2014.
- "CASSIAN (Katz), NINA". Retrieved 16 April 2014.
- Gray, Channing (19 June 2008). Poet, composer, refugee at URI, The Providence Journal[dead link]
- "NINA CASSIAN". Retrieved 20 April 2014.
- "Romanian Poet, Dissident Nina Cassian Dies". ABC News. 16 April 2014. Retrieved 17 April 2014.
- "Romanian poet, dissident Nina Cassian dies". The Washington Post. Retrieved 16 April 2014.
- Fox, Margalit (18 April 2014). "Nina Cassian, Exiled Romanian Poet, Dies at 89". The New York Times. Retrieved 20 April 2014.
- "Fabuloasele aventuri ale poetei Nina Cassian, "cea mai atrăgătoare femeie urâtă din literatura română"" (in Romanain). Adevarul. Retrieved 17 April 2014.
- Mălăncioiu, Ileana (9 November 2005). "Riscul de a privi memoria ca zestre". România literară (in Romanian). Retrieved 16 April 2014.
- Audio: Nina Cassian reads 'Epilogue' - poemsoutloud.net
- Desert Island Discs appearance (14 February 1999)