Diamond was the son of a biochemist and a fashion designer. He grew up in Lower Clapton and Woodford Green, he then attended the City of London School John Diamond Biography at lasting tribute .com and trained as an English teacher at Trent Park College of Education, now part of Middlesex University. Later he taught at an all-girls school before switching to journalism.

He wrote a regular column for the Saturday edition of The Times from 1992 onwards called "Something for the Weekend", and worked as a presenter on BBC radio and television. He met his second wife, journalist Nigella Lawson, when they were both writing for The Sunday Times. They married in Venice in 1992.

In 1997, Diamond was diagnosed with throat cancer. He wrote about his experiences with cancer in his newspaper column, for which he won the prestigious What The Papers Say award. In 1999 he was shortlisted for the Samuel Johnson Prize for his book C: Because Cowards Get Cancer Too.... A BBC documentary was filmed for Inside Story which followed him through various treatments, and showed his frustration with his speech difficulties following throat, and later tongue, surgery.

C: Because Cowards Get Cancer Too... was adapted into a play by Victoria Coren called A Lump In My Throat, which was itself later adapted for television. Diamond's second book, Snake Oil and Other Preoccupations, was edited by his brother-in-law Dominic Lawson, editor of the Sunday Telegraph, and published posthumously. It contained the six chapters of his "uncomplimentary look at the world of complementary medicine" which he had completed before his death, and some of his columns from The Times and the Jewish Chronicle.

Diamond was the recipient of the "HealthWatch Award" for 2000.

Bandolier reviewed and recommended his book C: Because Cowards Get Cancer Too...

Diamond died of throat cancer in Westminster, aged 47. On 3 September 2002, Nigella opened the John Diamond Voice Laboratory at the Royal Marsden Hospital in London, where he had been treated.