Judith Kazantzis was born in Oxford and grew up in East Sussex, the fourth child and second daughter of the eight children born to Lord and Lady Longford, and sister of novelist Rachel Billington and historians Dame Antonia Fraser and Thomas Pakenham. She attended St Leonards-Mayfield School, which she found stifling, and then More House School in Kensington, which she enjoyed. She wrote her first poem aged seven. She took a Modern History degree at Somerville College, Oxford. She began writing textbooks on history, worked for the Chelsea Labour Party and reviewed for the Evening Standard. She avoided the usage of the title "Lady" as the daughter of an earl. During the 1970s she turned to poetry, fiction, painting and printmaking. She was a committed feminist, writing for the magazine Spare Rib and was strongly influenced by Sylvia Plath's poetry.
In her own poetry she wrote about injustice and she contributed short stories to Critical Quarterly, which Plath had also written for. She supported the Campaign for Nuclear Disarmament and joined the protestors at RAF Greenham Common air base in the 1980s. She lived in London and later in East Sussex again, and spent three months a year in Key West where her second husband, Irving Weinman, taught.
In the 1990s she worked for Kalayaan - Justice for Migrant Domestic Workers. In 1999 she left Tony Blair’s Labour Party, and since 2001 helped to campaign for Occupied Palestine (she was a founder of British Writers in Support of Palestine). She chaired the judges of the Longford Prize (named in honour of her father) in support of prison reform. In 2003, she signed the Statement for Peace of the 21st Key West Literary Seminar. In August 2010 Kazantzis contributed to an eBook collection of political poems entitled Emergency Verse - Poetry in Defence of the Welfare State edited by Alan Morrison.
Her poems have appeared in London Magazine, Stand, Ambit, Agenda, Poetry Review, Poetry London, Poetry Wales, Bete Noire, The Honest Ulsterman, Poetry Ireland, Red Pepper, The Independent, The New Statesman, Tribune, and Banipal.
She married lawyer Alexander John Kazantzis on 26 February 1963 and had two children, Miranda and Arthur. The couple divorced in 1982, but she kept his surname professionally. On 22 February 1998, she married lawyer and writer Irving Weinman. Harry Mathews wrote an Epithalamium for Judith Kazantzis and Irving Weinman.
- "Tropic". Poemhunter.
- Just After Midnight. Enitharmon Press. 2004. ISBN 978-1-904634-02-7.
- The Odysseus Poems: Fictions on the Odyssey of Homer. Cargo Press. 1999. ISBN 978-1-899980-07-9.
- Swimming Through The Grand Hotel. Enitharmon Press. 1997. ISBN 978-1-900564-20-5.
- Selected Poems 1977-1992. Sinclair-Stevenson. 1995. ISBN 1-85619-552-X.
- The Rabbit Magician Plate. Sinclair-Stevenson. 1992. ISBN 978-1-85619-169-2.
- Flame Tree. Methuen. 1988. ISBN 978-0-413-17880-0.
- Poem for Guatemala. Bedlam. 1986. ISBN 978-1-870260-01-5.
- Freight Song (Poems on the Underground, tenth anthology). Cassell. 1986.
- Let's Pretend. 1984, Virago Press
- Touch Papers: Three Women Poets (with Michèle Roberts and Michelene Wandor. 1982, Allison and Busby).
- The Wicked Queen. Sidgwick and Jackson. 1980.
- Minefield. Sidgwick and Jackson. 1977.
- In Cyclops' Cave: The Odyssey, Book IX, LI 105-566. The Greville Press. 2002. ISBN 978-0-906887-74-5.
- Christmas Cards (Enitharmon Press, 2005)
- A Celebration of Wilfred Owen (The Interpreter's House)
- Poems On The Underground (Cassell, 2001)
- Parents (Enitharmon 2000)
- Red Sky At Night (Many Leaves Press, 2003)
- A Ring of Words (Arvon Prize Anthology, 1998)
- Mind Readings, Dancing in the Street, The Faber Book of Blue Verse and the Virago Book of Love Poetry.
- Poems on the Underground featured her poem "Freight Song" (Cassell).
- Michelene Wandor, ed. (1983). "The Errant Unicorn". On Gender and Writing. Taylor & Francis. ISBN 978-0-86358-021-5.
- "Judith Kazantzis, poet – obituary". The Daily Telegraph. London. 6 October 2018.
- "Statement for Peace". The New York Review of Books. 50 (2). 13 February 2003.
- The Recusant eZine, therecusant.org.uk; accessed 23 March 2016.
- Banipal magazine website Archived 2009-02-10 at the Wayback Machine, banipal.co.uk; accessed 23 March 2016.
- Arlo Haskell, "The Epithalamium of Harry Mathews" Archived 12 April 2009 at the Wayback Machine, Littoral, 29 May 2008.
- "Judith Kazantzis obituary". The Guardian. 23 October 2018.