The Cooks' own finances were little precarious, and when Ida obtained a contract with Mills and Boon to published her first novel in 1936, she left the Civil Service to write full time. As Mary Burchell, she became a prolific writer of romantic fiction. Her great popularity helped the success of Mills and Boon, and guaranteed substantial income after the war. For many decades, her writing supported her two passions: refugees and young opera singers. Her flat in Dolphin Square at various times housed homeless European families.

In 1950, Ida Cook wrote her autobiography: "We followed our stars", and in 1965, the Cook sisters were honoured as Righteous Gentiles by the Yad Vashem Martyrs and Heroes Remembrance Authority in Israel, thus joining Oskar Schindler among others.

She helped to found and was for many years president of the Romantic Novelist's Association. As Mary Burchell, she wrote over a hundred romance novels, many of which were translated, and her most famous work is "The Warrender Saga", a series about the opera world, full of real details. She also wrote as James Keene with William Everett Cook.

Ida Cook passed away on December 22, 1986 and her sister Louise in 1991.