Kessler was born to a Jewish family in London, England, and dropped out of school at the age of 15. Shortly thereafter, he wrote a screenplay that he showed to his mother's cousin, movie director Clive Donner. Although never produced, the screenplay made Kessler realize that he wanted to become a writer.

Kessler began writing at the age of 15 when he dropped out of school, but it wasn't until he was in his late thirties that he secured a publishing contract from Hodder Headline. His first book A Fool for a Client was a legal thriller and courtroom drama about the trial of a 23-year-old female medical student in New York for the murder of an Irish nationalist who had fled to the United States and avoided extradition. The title is an allusion to the legal aphorism that anyone who conducts their own defense “has a fool for a client.” In the novel, the accused, Justine Levy, insists on conducting her own defense despite judicial advice to the contrary. The book was favourably reviewed by Susanna Yager in the Sunday Telegraph, who wrote: “The author has thought up the most ingenious method of committing murder that I have come across in a long time.”

This was followed by The Other Victim about the stabbing of a teenager and the disappearance of a billionaire in what turn out to be linked cases. The London Express on Sunday wrote of this book: “The pace is fast, characters convince and the plot is well thought out. Kessler writes well."

However after his next two books,Tarnished Heroes and Reckless Justice, he was dropped by Hodder during a slump in publishing. At that same time, he courted controversy by co-writing a book about the murder of Rachel Nickell called Who Really Killed Rachel? with Colin Stagg the man who had been falsely accused of the murder and who at the time was still perceived by some to have been a guilty man who got away with the crime. The book—which was published in 1999 by small publisher Greenzone, after the major publishers turned it down—named Robert Napper as one of four credible suspects. Nine years later, Napper pleaded guilty to manslaughter due to diminished responsibility for the crime, after new DNA technology and other innovations in forensic science linked him to the crime.

Kessler made a comeback in 2009 when he signed a three book deal with the Avon division of HarperCollins UK for a series of books about a San Francisco lawyer, Alex Sedaka. Earlier, unpublished, versions of the first book in the series, Mercy, appeared on the Authonomy website (owned by HarperCollins) but it was not picked up by HarperCollins through Authonomy. Rather it was sent to HarperCollins by Kessler's agent and picked up through the conventional channels.

Mercy is about Sedaka's race against time to save a client on death row after the client declines an offer of clemency from the governor. The entire book takes place in a 15 hour time-frame and was described by James O'Brien on LBC as “a cracking thriller.”

No Way Out is about Sedaka's defense of a black neoconservative talk show host (and former black power activist) on a charge of raping a white girl in a cause celebre that has America bitterly divided.

Kessler cites Harlan Coben, Ayn Rand, Sidney Sheldon and Dan Brown as his influences. However, he also admitted that the inspiration for Mercy came from a conversation with a psychiatric nurse with whom he got talking when he called the wrong number while trying to contact an old friend.