Standing in the dock in an oversize black-and-red tracksuit, Dagorn was accused of killing Michel Kneffel, a man in his 60s with whom she had been living at a residential hotel in Nice and whose body was discovered in July 2011.
She was not charged at the time but the case was reopened the following year after police found vials of Valium and the personal documents of a string of men in among her belongings. These included bank account details, health insurance and identity cards.
She was also charged with murdering Francesco Filippone, 85, whose decomposing body was found in his bathtub in Mouans-Sartoux, outside Cannes, in February 2011.
Dagorn had earlier cashed a cheque from Mr Filippone for 21,000 - a "gift" she said was to help her open a jewellery shop.
Two other men she is accused of trying to poison will testify in court this week as civil plaintiffs.
One of them, 91-year old widower Robert Vaux, 91, said before the trial: "She was like a ray of sunshine in winter. When you are with a younger woman you know it won't last but you don't deny yourself the moment unless you're a masochist."
The retired sailor had brought Dagorn to live with him in early 2012 in the coastal town of Frejus, where he saw his health rapidly deteriorate.
"In the week when I was in a really bad way, she wrote two letters and sent a fax to my notary asking for custody of my affairs," he said. "At the time, even the chemist told me: 'Robert, you're in danger, you could a novel'."
Her lawyers said she denied all the charges. When initially charged over the cases in 2015, one of her lawyers, Georges Rimondi, described his client as a "fragile" individual who said "she feels better with elderly people". They said she had suffered from being placed in a foster home as a child.
Police now suspect Dagorn met at least 20 men after arriving on the French Riviera in 2011.
In most of the cases she allegedly asked them for money, stole documents from them or sought to have her name added to their wills.
"The life of Patricia Dagorn is marked by selfishness since her separation (with her ex-husband)," said a court profiler. "She only thinks of money to the detriment of her children."
Her youngest son in 2013 told local media he was not surprised at the accusations against his mother.
"She has always been obsessed with quick and easy money," the son, known only as Guilhem, told Nice Matin.
The case bears marked similarities to the "Black Widow of Isère" affair, which in 2014 saw a woman sentenced to 30 years in jail for drugging her husband and burning him to death.
Investigators looking into the past of Manuela Gonzalez, from the French Alpine Isère region, discovered that four of her previous partners had been poisoned. Two died.
