Stewart was born in Sunderland, County Durham, England and graduated from Durham University, from where she received an honorary D.Litt in 2009. She was a lecturer in English Language and Literature there until her marriage in 1945 to Sir Frederick Stewart, former chairman of the Geology Department of Edinburgh University. Sir Frederick died in 2001.

She is also the author of many crime fiction, gothic fiction and romance novels, several of which have been adapted for television and/or film. Several of her books are set in Scotland; others are set in more exotic locations such as Damascus, the Greek islands, Spain, France, Austria, etc.

She was at the height of her popularity in the 1960s and 1970s when many of her suspense and romantic novels were translated into many languages.Stewart is considered one of the founders of the romantic suspense subgenre, blending romance novels and mystery. Her novels seamlessly combined the two genres, maintaining a full mystery while focusing on the courtship between two people. In her novels, the process of solving the mystery "helps to illuminate" the hero's personality, helping the heroine to fall in love with him.

Following the success of T.H. White's The Once and Future King, and the connection of the Kennedy presidency with "Camelot", Arthurian Legends became quite popular again. Mary Stewart added to this climate by publishing The Crystal Cave , the first in what was to become a 5 book series later dubbed The Merlin Chronicles. It placed Lady Stewart on the best seller list many times throughout the 1960s,1970s, and 1980s.

The Merlin Series

1. The Crystal Cave (1970)
2. The Hollow Hills (1973)
3. The Last Enchantment (1979)
4. The Wicked Day (1983)
5. The Prince and the Pilgrim (1995)

Others

• Madam, Will You Talk? (1954)
• Wildfire at Midnight (1956)
• Thunder on the Right (1957)
• Nine Coaches Waiting (1958)
• My Brother Michael (1959)
• The Ivy Tree (1961)
• The Moon-Spinners (1962)
• This Rough Magic (1964)
• Airs Above the Ground (1965)
• The Gabriel Hounds (1967)
• The Wind Off the Small Isles (1968)
• The Little Broomstick (1971) (children's novel)
• Ludo and the Star Horse (1974) (children's novel)
• Touch Not the Cat (1976)
• A Walk in Wolf Wood (1980) (children's novel)
• Thornyhold (1988)
• Frost on the Window: And other Poems (1990) (poetry collection)
• Stormy Petrel (1991)
• Rose Cottage (1997)