Christy Norman, 70, from Poole, Dorset, told a court she dialled 999 and performed CPR while everyone else fled.
The English Collective of Prostitutes (ECP), which supported her, said she was only a cleaner working two days a week at the brothel in Bournemouth.
Norman denied assisting in the brothel's management but was found guilty at Poole Magistrates' Court.
She was given a conditional discharge and ordered to pay Â£400 court costs.
Cari Mitchell, from the ECP, said the "traumatised" defendant was only guilty of trying to save a life.
Ms Mitchell, who was present in court, said: "It is a terrible injustice that a woman who performed a civic duty by trying to save another human being should find herself with a criminal record.
"Shame on the police, the Crown Prosecution Service (CPS) and the judge.
"If they had any morals or were in anyway concerned for women's safety this prosecution would never have been brought."
The man had asked to come and sit in the Bunnies Ranch premises on 19 May 2016 after feeling unwell, the court heard.
He then collapsed and Norman spent eight minutes performing CPR before paramedics and police officers arrived. He was pronounced dead at the scene.
Norman was later arrested and charged.
In a statement, the CPS said Norman was "clearly involved in the running and management of the brothel".
District prosecutor Tim Cole said: "She had been publicising the Ranch's services on local sex sites and was keeping a book where she would record the sexual practices of the employees with their clients."
Norman, of Arley Road, was "too upset to comment", the ECP said.
A woman who tried to save a man's life at a brothel has been found guilty of helping to run the establishment.