Marion Walsh was born in 1910. Her father worked for Standard Oil of New Jersey in Bucharest. She married Alfred Gardyne de Chastelain, a petroleum engineer and sales manager with Unirea de Petrol Company in Romania.
On the outbreak of the Second World War she returned to New York City in November 1940. Soon afterwards she obtained work with William Stephenson, the head of British Security Coordination (BSC) as a cipher clerk. Soon afterwards she was appointed as courier/translator for BSC agent, Elizabeth Park. "She was the type who reveled in espionage. She really loved it. And she came from a good Washington family so she had entree to all the embassies and places... She was tall... a dark blond... beautiful figure... not terribly good-looking, but she... certainly appealed to the males."
In November 1941, Japanese special envoy Suburu Kurusu, arrived in the United States. One of the BSC agents was able to record his conversations. On 27th November, 1941, William Stephenson sent a telegram to the British government: "Japanese negotiations off. Expect action within two weeks."
The attack on Pearl Harbor took place on 7th December, 1941. Marion de Chastelain was one of those transcribing these conversations. She later recalled: "If they were monitoring a lot, it is hard to see how they missed it. My opinion is that they did know, and unfortunately the poor admiral and general in Pearl Harbor took the blame."
Marion moved to London in November 1943: "I worked for Section 5 of MI6. That's counter-intelligence, and we were in the Charity Commissioner's office building... on Ryder Street, just off St. James Street." She worked with Hugh Trevor-Roper and Kim Philby: "Trevor-Roper and Philby were up in the attic above my head... Hugh Trevor-Roper is a little bit of a man with a big ego."
Marion de Chastelain used to see William Stephenson and his wife when they moved to Bermuda. "Mary didn't particularly care for Bermuda... She loved New York and she had lots of friends.... she found Bermuda fairly boring... It must have been difficult for her, because Bill was not a man to socialize. You know, go to big parties." She objected to an article in a magazine by David A. Stafford that suggested Stephenson was senile by this time: "He wasn't out of it at all. The impression of course could be due to his speech problem (after his stroke). Sometimes it was extremely good. And other times it wasn't... that would give the impression that he wasn't quite with it. You had to listen to what he said, not the way he said it."
Marion de Chastelain died in 2000.
I have appointed Mr W.S. Stephenson to take charge of my organisation in the USA and Mexico. As I have explained to you, he has a good contact with an official who sees the President daily. I believe this may prove of great value to the Foreign Office in the future outside and beyond the matters on which that official will give assistance to Stephenson. Stephenson leaves this week. Officially he will go as Principal Passport Control Officer for the USA. I feel that he should have contact with the Ambassador, and should like him to have a personal letter from Cadogan to the effect that it may at times be desirable for the Ambassador to have personal contact with Mr Stephenson.