Her father, who was a medical doctor, died of influenza due to tending many patients after World War I. Mollie grew up under the care of her grandparents.
While working in London in the 1930s, she met her future husband, Orval John Gillen, a warrant officer with the Royal Canadian Air Force Headquarters Staff then stationed in England.
In 1961, she moved to Toronto and eventually became associate editor and staff writer for the Canadian magazine Chatelaine.
Her research led her to write and publish many books on various topics important to Canada and Australia including: The Masseys: Founding Family (1965), The Prince and His Lady (1970), The Assassination of the Prime Minister (1972), The Wheel of Things: A Biography of L.M.
[9] Dr. Gillen lived in Dolphin Square in London for many years before returning to Toronto, Ontario, Canada.