Sarah Hinkson, 37, draped herself across four seats on the Hammersmith and City line while touching herself and moaning loudly in February.
Hinkson’s hand was allegedly still down her trousers when she was arrested by police officers at King’s Cross station.
Passenger Anthony Burton – who is visually impaired in one eye – told City of London Magistrates’ Court he boarded at Paddington after being escorted into the front carriage with his guide dog.
He said: “I was suddenly aware of a banging noise and I thought it was something mechanical but I noticed that to my right there was a lady who was laying out across three or four seats.
“She was banging her foot against the seat or the glass which was making the noise.
“There was not anybody standing in the way, I could see her quite clearly.”
Mr Burton denied his impaired vision had affected what he had seen.
He continued: “All I could see of her hand was the top of her wrist area because the rest of her hand was in her trousers.
“It was around her crotch, it was blatantly obvious which part of her body it was.
“Her hands were moving back and forth in a sort of gyratory movement.
“I was suddenly aware of a banging noise”
Passenger Anthony Burton
“She wasn’t saying anything but there were moans and groans of a sexual nature. I was quite taken aback.”
Mr Burton contacted the police after seeing there were children nearby at risk of seeing.
Hinkson later told officers: “Rash on my inner thigh, I was scratching a rash on my inner thigh. It comes and goes.”
Hinkson, from Acton, west London, denied outraging public decency by behaving in an indecent manner but was convicted.
Chair Magistrate Diane Lennon found Hinkson guilty, stating: ‘We were asked to consider an offence of outraging public decency.
“Mr Burton clearly saw indecent acts taking place over a material period of time.
“We find the Crown has proven this incident occurred beyond all reasonable doubt.”
She did not attend court and a warrant has been issued for her arrest.