At 4 feet, 7 inches, the 86-year-old Jewish grandmother hardly looks like the James Bond-type. But during World War II, Cohn, then in her 20s, worked as an undercover agent with French intelligence.
Speaking in a heavy French accent, Cohn relayed her story -- which she has chronicled in her autobiography Behind Enemy Lines -- to mesmerized listeners at the Center City Gershman Y.
Born in 1920, Cohn grew up Lorraine, in the northeast corner of France, where many residents also spoke German. Though the blonde-haired, blue-eyed young woman didn't look Jewish, she described her family as very religious; her grandfather had been an Orthodox rabbi.
The war began when Cohn was 19. She said that, from the start, all six of her siblings joined resistance activities.
"We were a fighting family," she told the crowd.
In 1941, at the urging of her boyfriend, Cohn enrolled in the local Red Cross nursing college.
But she left school when her father and sister were arrested by the SS. Though her father was released later that same day, Stéphanie continued to be held. Cohn worked hard to get her released, but to no avail -- her sister was later deported to Auschwitz, where she died.
Tragedy struck again in 1943, when Cohn's boyfriend, now her fiance, was executed for killing a leading Nazi collaborator.
Heartbroken and seeking a fresh start, Cohn, who had by this point completed her nursing degree, enlisted in the French army.
Though she signed on to be a nurse, Cohn started as a social worker. She said that she spent most of her time running in and out of foxholes -- wearing a uniform three sizes too large -- and taking orders for supplies.
Cohn said she became a spy purely by chance: She happened to mention to a colonel that she spoke fluent German. He quickly reassigned her to intelligence.
It was there that Cohn learned how to fire a gun, interpret military code and decipher Nazi troop movements. She used these skills to interrogate German POWs and sniff out German spies.
Cohn's most harrowing missions forced her into Germany. Traveling alone -- and often at night -- she called the prospect of a border crossing "terrifying."
"I felt like I was paralyzed," she said. "I really couldn't make myself go into Germany."
But go she did, several times, using the alias Martha Ulrich, a nurse looking for her German fiance. There, she discovered an imminent ambush in the Black Forest and about the abandonment of a key German post; both proved to be crucial tips.
For many years, Cohn didn't reveal this story to anyone -- not even her husband or two sons: "I always felt that people would think they were tall tales."
In 1996, responding to an ad for stories of French Jewish resistance, Cohn starting talking.
At age 80, she won France's highest military honor, the Médaille Militaire, and began writing her autobiography.
Today, the California resident speaks regularly about her wartime activities. "Fighting was very important; we did not want to let them get the best of us. I regret only that I didn't do more."