Marsali grew up near Edinburgh, Scotland. Her summer family holidays were spent in a remote cottage in the West Highlands, the region where her detective Gavin Macrae lives. Like her sailing heroine, Cass, she has always been used to boats, and used her 'gap year' earnings to buy her first sailing dinghy, Lady Blue. She studied English at Dundee University, did a year of teacher training and took up her first post, teaching English and French to secondary children in Aith, Shetland. Gradually her role expanded to doing drama too, and both primary and secondary pupils have won prizes performing her plays at the local Drama Festival. Some of these plays were in Shetlandic, the local dialect.

Marsali teaches dinghy sailing at her local club, and is a keen single-handed sailor in her Offshore 8 m yacht, Karima S - the double of Cass's Khalida.

A qualified STGA green badge tourist guide for Shetland, she now spends a good deal of her summer sharing her home with visitors from overseas. She is particularly interested in women's history, and has published Women's Suffrage in Shetland, two years' worth of original research. She followed this with The Story of Busta House, the romantic tale of the house which is the setting for part of Death on a Longship.

Marsali also writes for the monthly magazine Shetland Life - a mixture of travel writing, interviews, investigative journalism and historical research.