Birbalsingh was born in New Zealand, the elder of two daughters of Frank Birbalsingh, a teacher, and his wife Norma, a nurse from Jamaica.
Birbalsingh's father and grandfather were both educators. Her paternal grandfather, Ezrom S. Birbalsingh, was head of the Canadian Mission School in Better Hope, Demerara, Guyana. Her father (born 1938 in Berbice, Guyana) obtained his MA in English in London in 1966, specializing in Commonwealth literature, and worked as a supply teacher in Birmingham and London.
Frank moved to Toronto, Canada, in 1967, where he worked again as a supply teacher, joined the faculty at York University in 1970, and obtained his PhD in Canadian literature in 1972. He held several other positions over the years, including a fellowship at the University of Delhi, India, and a postdoctoral research fellowship at the University of Auckland, New Zealand (1973–1974), where Birbalsingh was born.
The family lived in England when Frank was a visiting fellow at the Centre for Caribbean Studies, University of Warwick (1989–1990). In 1996 he was promoted to professor at York and in 2003 became professor emeritus. Birbalsingh grew up mostly in Toronto, but moved to the UK at age 15 when her father was teaching at Warwick. When the family returned to Canada, she decided to stay in the UK. She graduated from Oxford University after reading French and philosophy at New College.
While at Oxford Birbalsingh had visited inner-city schools, as part of a scheme the university runs, to encourage state-school pupils to apply, and after graduation she decided to teach in state schools herself. From 2007 she wrote an anonymous blog, To Miss With Love, in which—as Miss Snuffy—she described her experiences teaching at an inner-city secondary school. In 2010 she was the assistant head of Dunraven School, Streatham, south London, and that year she joined St Michael and All Angels Academy in Camberwell, also south London, as vice-principal.
Birbalsingh is a supporter of the traditional teaching methods described in E. D. Hirsch's The Schools We Need and Why We Don't Have Them (1999). She writes that the book "opened [her] eyes" to what was wrong in schools, and argues that education should be about teaching children knowledge, not learning skills. Responding to the removal of Michael Gove as education secretary in 2014—Gove was also a supporter of Hirsch—she said it was a tragedy that his work would not be completed.
Birbalsingh came to national prominence in October 2010 after criticising the British education system at that year's Conservative Party conference, and speaking in support of the party's education policies. Referring to a "culture of excuses, of low standards ... a sea of bureaucracy ... [and] the chaos of our classrooms", Birbalsingh told the conference: "My experience of teaching for over a decade in five different schools has convinced me beyond a shadow of a doubt that the system is broken, because it keeps poor children poor." She said that as a result she became the target of racist and sexist abuse on social media.
After the speech Birbalsingh was asked not to attend her school while the governors "discuss[ed] her position". She subsequently resigned "after being asked to comply with conditions that she did not feel able to comply with", according to The Sunday Telegraph.
Birbalsingh's first publication was a chick-lit novel, Singleholic (2009), published under the pseudonym "Katherine Bing". Her second book, To Miss with Love (2011), was based on her blog. It was chosen as Book of the Week and serialised on BBC Radio 4. She is also the editor of Battle Hymn of the Tiger Teachers: The Michaela Way (2016), which describes the education philosophy of Michaela Community School.(2016) Battle Hymn of the Tiger Teachers: The Michaela Way, John Catt Educational. ISBN 978-1909717961
(2011) To Miss with Love, Penguin. ISBN 978-0670918997
(2009) Singleholic (writing as Katherine Bing), Hansib Publications. ISBN 978-1906190156