Sandra Muller was ordered to pay €20,000 ($22,000, £17,500) in damages and fees to ex-TV boss Éric Brion, who she said had flirted with her inappropriately.
She created the “#balancetonporc” (“rat on your pig”) hashtag in 2017.
Ms Muller said she would appeal.
“I lost today, but the other women won,” she said following the verdict on Wednesday.
Ms Muller was ordered to delete a tweet she posted in 2017 which named Mr Brion and told to post statements on her account that were issued by the cour, BBC reported.
Her lawyer, Francis Szpiner, said the verdict in the civil case was “outdated”.
During a hearing in May, Mr Brion’s legal team said that while he had apologised for his clumsy or inappropriate flirtation, he had “never admitted to harassing anyone”.
“He said that one evening he tried to flirt with Sandra Muller as he liked her,” his lawyer Marie Burguburu told the court. “This is his right to flirt,” she added.
Ms Muller, who had travelled from the United States, where she is based, to attend the session, said the “humiliating” incident prompted her to start “a movement that spread through all levels of society”.
She defended her right to freedom of expression and said her tweets were aimed at encouraging others to share their experiences, adding that sexist insults needed to be taken seriously, BBC reported.
But Mr Brion described the “machine” Ms Muller launched with her tweets as “unstoppable”, and alleged that he had been subject to serious professional and personal repercussions because of it.
Following news of Mr Brion’s legal action, Ms Muller wrote on Facebook: “I will go to the end of this fight with the help of my lawyer and I hope that this trial will be an opportunity for a real debate on how to combat sexual harassment.”
In October 2017, Ms Muller tweeted: “#balancetonporc! You too can recount by giving the name and details of sexual harassment you have known in your job.”
Just a few hours later, she followed up with: “You have big breasts. You are my type of woman. I will make you orgasm all night” along with Mr Brion’s name and position.
But Mr Brion said he had never worked with Ms Muller in any capacity; that he had apologised for his words, and not pursued it after he was rejected.
The resulting social media campaign behind the hashtag, however, spread Mr Brion’s name far and wide, and in December that year, he wrote a response in French newspaper Le Monde.
He said he made his “inappropriate remarks” at a cocktail party late in the evening, “but only once” – and said he was not trying to excuse his actions. He also said there was a need for “truth and nuance” amid the wave of accusations.
He added that Ms Muller had implied the two had some sort of working relationship in which harassment took place.
He suggested she had been deliberately vague, in a “conflation of heavy-handed flirting and sexual harassment in the workplace”.
As a result, he said it had become extremely difficult to find work, according to BBC report.
Mr Szpiner, Ms Muller’s lawyer, told the court in May: “If you do not feel that when a man stands in front of you and makes you such a proposal, it’s offensive, I’m sorry for you, but today a majority of women and men think it is.”
“If this hashtag has been successful it’s because thousands of women have recognized themselves [in it],” he added, in remarks reported by France’s 20 Minutes programme.