Women’s History Month runs from the 1st to 31st March each year since 1987. Each week Horus Security Consultancy will be exploring the work of notable women in security that have made significant contributions to the industry throughout history. From WW2 intelligence officers to the first female Director General at MI5. This week we are sharing the life and career of MI5’s first female agent runner, Mary Sherer. 

Mary Sherer

Mary Sherer joined MI5 in 1936 as a secretary at the age of 21. After a posting as a PA in Malta, she returned to London on the outbreak of war as an assistant to Lord Rothschild, the nation’s leading authority on German and Italian sabotage methods.

Sherer was later promoted to a desk officer, and in 1942 was posted to Washington as an adviser on sabotage to the Head of British Intelligence and Security in North America. 


Double Cross

Sherer returned to London the following year and joined the section responsible for turning German agents back against the Abwehr (the German Military Intelligence Service), known as Double Cross.  

She had sole responsibility for the agent known as TREASURE, a French national and talented artist named Lily Sergeyev, who had volunteered her services to the Germans with the intention of double-crossing them. After the Abwehr took up her offer, she contacted the British Embassy in Madrid and was flown to London, where she was debriefed, and soon after recruited by Sherer, who was convinced that TREASURE would be a successful double agent.    

TREASURE worked under Sherer’s direction as a deception agent, feeding a mixture of false and genuine information back to Germany. However, the relationship faced numerous difficulties. TREASURE had left her beloved dog, Babs, in Gibraltar due to British quarantine rules, but was intent on bringing Babs to the United Kingdom.  

In 1943, she told Sherer she would refuse to send any further messages to the Abwehr until they had been reunited. TREASURE relented a few months later and restarted communication with the Germans, but the situation deteriorated again when the unfortunate Babs died in Gibraltar, a few weeks before D-Day.  

TREASURE confessed to Sherer that she had hatched a plan to wreak revenge on the British for Babs’ death by warning her German handlers via a code that she could insert in her transmissions to indicate that she had been compromised but would not tell Sherer what the code was.  

The Double Cross deception, designed to mislead the Germans on the D-Day landing locations, was reliant on TREASURE continuing to send bogus messages to her German handlers. Considering TREASURE’s confession, Sherer was faced with a major dilemma: pull the plug on TREASURE’s communications with the Abwehr and risk the Germans becoming suspicious or continue with the risk that TREASURE might insert a code to warn them in future messages.  

The decision was taken to keep TREASURE in play, and her communications contributed towards the successful D-Day Double Cross deception. Shortly afterwards, TREASURE was discontinued as an agent and returned to France later in 1944. Before she left, Sherer managed to get TREASURE to reveal what the warning code would have been to the Germans. Sherer subsequently checked every message that had been sent to the Abwehr and confirmed that TREASURE had never used it.  

Sherer was highly commended for her contribution to the success of Double Cross but, as a woman, she was never given officer rank alongside her male counterparts. She chose to leave MI5 after the war and moved to Rome with Phyllis McKenzie, who had worked for British Intelligence in New York during the war, where they opened a bookshop.   

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