She got away with it for 12 years before she ever got caught — but that didn't stop her from doing it again.
Her simple trick, she revealed, was to lock onto another passenger at the airport and just follow them through as if you were with them.
"I have never been able to board a plane by myself. I was always let through," she revealed. "I mean I was able to go through the security line without a boarding pass."
"I got by them, this is the thing that is so crazy, by following someone they would be carrying like a blue bag," she explained. "And the next thing I know, I get into the TSA line and TSA lets me through, and they think I'm with the guy with the blue bag."
Without ever paying for a ticket, she has flown all over the US, including Hawaii and Alaska, and has crossed the Atlantic to Denmark, England and Spain.
Her first of several arrests came in 2014; since then she has been described in various court documents as a "serial stowaway", "habitual stowaway" and "a high security risk"; she features on many airlines' "trespass list", and her picture is posted at TSA checkpoints in airports across the country.
But still she managed to slip through: in January 2018 she managed to board a $3,428 British Airways flight to London from Chicago's O'Hare; she was arrested on the far side and sent back when she was discovered without documentation.
While out on probation for this offense, she was arrested in October 2019 when TSA caught her trying to board yet another flight from O'Hare ticketless.
Hartmanspent a year and a half in jail for the latest charges of burglary, criminal trespassing, and probation violation, before being released pending electronic monitoring, counseling and medication.
Having been diagnosed bipolar, she was "determined by forensic psychologists to be incompetent to proceed" following her first court appearance in 2015.
However, the judge's patience with her seems to be running out; her attorney's latest plea offer, which will discover its fate in April, would see her burglary charge dropped in lieu of 18 months probation and mental health counseling. But the judge said at the hearing "I will not be giving her a third probation."
"I have real tough judge on this case," Hartman lamented. "Judge Chiampas is pissed at me for the situation in which I found myself."
Jeff Price, former Assistant Head of Security at Denver International Airport and author of "Practical Aviation Security: Predicting and Preventing Future Threats", believes there should be an entire curriculum dedicated to Marilyn Hartman for every transport security officer in training.
"For her to be able to repeat that over and over, that is just mind boggling," he told CBS.
"We definitely need to think differently," he added. "We can't constantly be preparing for the last attack. The next attack will not look like that."
As for Hartman, she just wants the opportunity to apologize to those she's hurt.
"I don't care at all if someone calls me crazy," she said. "I mean the story is crazy."