Soon after arriving in England Lefort joined the Special Operations Executive (SOE). On 16th June, 1943, she was given the code name "Alice" and sent to occupied France with Noor Inayat Khan and Diana Rowden. After she arrived she joined the Jockey Network led by Francis Cammaerts.
By the autumn of 1943, Francis Cammaerts had established a network of small independent groups up and down the left bank of the Rhone Valley. He developed a secure system where although he knew how to get in touch with members of the group, they had no idea where he was living and could only leave messages for him in letter boxes (somebody with whom one could leave a message to be collected later by another person giving the right password).
In September 1943 Lefort was arrested while visiting the house of a corn-merchant at Montélimar. She was tortured by the Gestapo but the system Cammaerts had set up enabled the Jockey Network to survive. Cecily Marie Lefort died of starvation or was executed at the Ravensbruck Concentration Camp in early 1945.