thumb|Marion de Chastelain in 1945
Marion Elizabeth (Walsh) de Chastelain was an American-born Canadian who worked as an agent for the British Secret Intelligence Service during World War II, passing on information from German-controlled Vichy French embassy to the Allies.
De Chatelain was born in Freehold, New Jersey, USA, on 24 May 1910. The daughter of a businessman working for Standard Oil of New Jersey in Romania, she was educated in Switzerland and at the Sorbonne, from where she graduated with a degree in international law at the age of 21.
A linguist, she married Alfred George Gardyne de Chastelain in Bucharest in the early 1930s and they had two children, Jacqueline, born in 1936 and John, born in 1937, who would become a well-known general in the Canadian Forces. Marion knew how to speak 7 languages, which helped her job as a spy become easier.
At the outbreak of war between Great Britain and Germany in 1939, she took her children to stay with relatives in England and returned to Romania. In 1940 she returned to England and took the children by sea to live with her parents in New York, USA. There she was recruited to work for Canadian Sir William Stephenson (to whom Winston Churchill had given the codename "INTREPID"), travelling regularly to Washington, D.C. to debrief agents working on behalf of the allies for Stephenson. Working with Amy Elizabeth Thorpe, De Chastelain was able to establish covert relationships between British and American intelligence agencies.