Shortlisted for a Historical Writers' Association Gold Crown Award, the story of Grace Darling had fascinated Gaynor from a young age. Gaynor's October 2018 novel The Lighthouse Keeper's Daughter was a USA Today and Irish Times bestseller. In August 2017, Gaynor published her first fantasy novel The Cottingley Secret, a fictionalisation of the Cottingley fairies story from 1917 Yorkshire.

This was followed by The Girl from the Savoy on London's interwar period in June 2016, a Globe and Mail and Irish Times best seller shortlisted for the Irish Book Awards. It appeared in WHSmith's spring selection. The character of Albert Shaw is based on John Grooms. Her second novel A Memory of Violets, originally titled Daughters of the Flowers, was published in February 2015. It became a New York Times and USA Today bestseller and was named the Romantic Novelists' Association Historical Novel of the Year. In 2013, she landed her first publishing deal with HarperCollins under the William Morrow imprint, who re-published the novel in print in 2014. She received the Cecil Day Lewis Literary Bursary Award at Kildare Readers' Festival that year. Gaynor self-published her debut fiction novel The Girl Who Came Home, based on the true story of 14 Irish emigrants on the Titanic, on Kindle in 2012. She contributed to and interviewed authors such as Philippa Gregory for the likes of Writing.ie. She ran a personal blog titled Hot Cross Mum, which became an e-book in 2011. After the 2008 recession left her redundant from her job as a learning and development executive at the Irish law firm A&L Goodbody, she decided to pursue her passion of writing. Gaynor began her career in professional services.

They settled in Kilcullen, County Kildare in 2004 where they have raised their two sons. She moved to Ireland in 2001 after meeting her now husband through a mutual friend in Dublin. She lived in Manchester, Australia, and London in the 1990s. She and her sister lost their mother in 1995. She graduated in 1993 with a Bachelor of Arts in Business Studies from Manchester Metropolitan University. Gaynor is from Driffield, East Riding of Yorkshire. Hazel Gaynor (born ) is an English author of historical fiction and fantasy based in Ireland.

Meet Me in Monaco by Hazel Gaynor