Justina was born in Yorkshire, England, in 1968. After completing school, she dropped out of Art College, then studied philosophy and linguistics at York University. She sold her first novel in 1999. Since then she has won the 2000 Amazon UK Writers' Bursary Award. She has also been a student (1992) and a teacher (2002, 2006) at the Arvon Foundation in the UK, a center for the development and promotion of all kinds of creative writing. She was a student at Clarion West, the U.S. boot camp for science fiction and fantasy writers, in 1996.
Her books have been variously shortlisted for the British Science Fiction Best Novel Award, the Arthur C. Clarke Award, the Philip K. Dick Award, and the John W. Campbell Award. An anthology of her short fiction, Heliotrope, was published in 2012. In 2004, Justina was a judge for the Arthur C. Clarke Award, on behalf of the Science Fiction Foundation.
Her novels and stories range widely over science fiction and fantasy, often in combination. She still lives in t'North of England with her partner and three children and a cat.