Mary, Lady Stewart (born Mary Florence Elinor Rainbow; 17 September 1916 – 9 May 2014) was a British novelist who developed the romantic mystery genre, featuring smart, adventurous heroines who could hold their own in dangerous situations. She also wrote children's books and poetry, but may be best known for her Merlin series, which straddles the boundary between the historical novel and fantasy.

Mary Florence Elinor Rainbow was born on 17 September 1916 in Sunderland, County Durham, England, UK, daughter of Mary Edith Matthews, a primary school teacher from New Zealand, and Frederick Albert Rainbow, a vicar.

She was a bright child and attended Eden Hall boarding school in Penrith, Cumbria, age eight. She was bullied there and stated that this had a lasting effect on her. At ten, she won a scholarship to Skellfield School, Ripon, Yorkshire, where she excelled at sport. Offered places by Oxford, Cambridge, and Durham universities, she chose Durham as it offered the largest bursary and least travel.[

She graduated from Durham University in 1938 with first-class honours in English, was awarded a first-class Teaching Diploma in English with Art the following year and in 1941 gained her master's degree.

The scarcity of jobs during World War II meant that she held a variety of posts during this period, including primary school teaching, teaching at secondary level at a girls' boarding school, and working part-time at the Sixth Form of Durham School. Between 1941 and 1956, she was an assistant lecturer (1941–5) and part-time lecturer (1948–56) in English literature, mostly Anglo-Saxon, at Durham University. She received an honorary D.Litt. in 2009.[5] It was in Durham that she met and married her husband, Frederick Stewart, a young Scot who lectured in Geology. She became known as Mary Stewart.

In 1956, the couple moved to Edinburgh. Mary, in her own words, was a "born storyteller" and had been writing stories since the age of three. Following the move to Scotland, she submitted a novel to the publishers Hodder & Stoughton. Madam, Will You Talk? was an immediate success, followed by many other successful works over the years.

Stewart was the best-selling author of many romantic suspense and historical fiction novels. They were well received by critics, due especially to her skillful story-telling and elegant prose. Her novels are also known for their well-crafted settings, many in England but also in such locations as Damascus and the Greek islands, as well as Spain, France, Austria, etc.

She was at the height of her popularity from the late 1950s to the 1980s, when many of her novels were translated into other languages. The Moon-Spinners, one of her most popular novels, was also made into a Disney live-action movie. Stewart was one of the most prominent writers of the romantic suspense subgenre, blending romance novels and mystery. Critically, her works are considered superior to those of other acclaimed romantic suspense novelists, such as Victoria Holt and Phyllis Whitney. She seamlessly combined the two genres, maintaining a full mystery while focusing on the courtship between two people, so that the process of solving the mystery "helps to illuminate" the hero's personality—thereby helping the heroine to fall in love with him.

In the late 1960s a new generation of young readers revived a readership in T. H. White's The Once and Future King (published in full 1958) and The Lord of the Rings (published in full 1956), and as a consequence Arthurian and heroic legends regained popularity among a critical mass of readers. Mary Stewart added to this climate by publishing The Crystal Cave (1970), the first in what was to become The Merlin Trilogy, later extended by two further novels. The books placed Stewart on the best-seller list many times throughout the 1970s and 1980s.