Bradley had been interested in writing his entire life. However, he began to take it more seriously in his early 30s after moving to Saskatoon. There, he joined several writing groups and began spending time with local writers.

He wrote several short stories that were read on CBC Radio and published in literary magazines. Following his early retirement from the University of Saskatchewan in 1994, Bradley and his wife Shirley moved to Kelowna, British Columbia for her work, while Bradley focused full-time on writing. He wrote multiple screenplays over the course of nine years. Then, during the 2003 Okanagan Mountain Park Fire, many homes neighbouring Bradley's were destroyed, although his was spared from the fire. The experience inspired him to do something different, and he began focusing on memoirs instead of screenplays. He wrote a non-fiction book called Ms Holmes of Baker Street and a memoir called The Shoebox Bible. In 2006, he began writing his first novel.

Soon after, Bradley's wife was listening to CBC Radio as Louise Penny, a Canadian mystery author, discussed the Debut Dagger fiction competition, run by the U.K. Crime Writers' Association and sponsored by the Orion Publishing Group in Britain. The competition requires that entrants submit the first chapter and a synopsis of a murder mystery. Bradley's wife encouraged him to write something new about the "girl on the camp stool", a minor character that had emerged in the novel Bradley was working on. In early 2007, Bradley entered the Dagger contest by submitting fifteen pages about the "girl on the camp stool" character, now named Flavia de Luce. These pages, which took only a few days to write and several weeks of polishing, would become the basis of The Sweetness at the Bottom of the Pie. Bradley set the book in England despite having never visited it. In June 2007, two judges from the contest contacted Bradley's agent in Canada to express interest in publishing the proposed book; they also inadvertently informed him that Bradley was the winner of the competition. A bidding war ensued, and on June 27, 2007, Bradley sold Orion the rights for three books in Britain. Within several days, Doubleday had purchased the Canadian rights and Bantam Books the U.S. rights. At age 69, Bradley left North America for the first time when he went to London, England to pick up the Dagger award on July 5, 2007.

Upon his return to Canada after the award ceremony, Bradley took a few weeks off, and then spent seven months turning the submitted fifteen pages into a full-length novel. The Sweetness at the Bottom of the Pie was published in the UK in January 2009 and in Canada in February 2009. The book has since developed into a series of novels about young Flavia de Luce solving various crimes in a 1950s village. The second installment (The Weed That Strings the Hangman's Bag) was published in March 2010, the third (A Red Herring without Mustard) in February 2011, the fourth (I Am Half-Sick of Shadows) in December 2011, and the fifth (Speaking from Among the Bones) in January 2013. The sixth book, The Dead in Their Vaulted Arches, was released in early 2014. The series has been extended to ten books, up from an original order of six.

In 2012, director Sam Mendes optioned the Flavia de Luce series, which he intends to develop into TV movies.

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