New Zealand author known especially for her many detective novels featuring Inspector Roderick Alleyn of Scotland Yard, and, in later novels, his wife Troy.

Marsh studied painting in art school and was an actress and theatrical producer in New Zealand before going in 1928 to England, where she wrote her first novel, A Man Lay Dead (1934), which introduced the detective Roderick Alleyn. In 1933 she returned to New Zealand, where she wrote many more novels and also produced and directed Shakespearean repertory theatre. The theatre guild she helped found in 1944 became an important mainstay of New Zealand cultural life.

In the 1930s Marsh helped raise the detective story to the level of a respectable literary genre by writing books that combine an elegant literary style with deftly observed characters and credible social settings. The art world and the theatre provided the background for many of her more than 30 novels, including Artists in Crime (1938), Final Curtain (1947), and Opening Night (1951), all of which feature Inspector Alleyn. These books, together with such works as Overture to Death (1939), A Surfeit of Lampreys (1941), Death of a Fool (1956), Dead Water (1963), and Black as He's Painted (1974), are classic examples of the traditional detective story, giving readers a cleverly contrived puzzle involving sharply drawn characters against an authentic background. Marsh also wrote about New Zealand and about the theatre, and her autobiography, Black Beech and Honeydew, was published in 1965. She was made a Dame Commander of the Order of the British Empire in 1966.