Borne previously served as minister of environment, transport and labor.
He will lead a French government tasked with fulfilling President Macron’s election campaign priorities: pension reform and expanding policies designed to tackle climate change.
His appointment came after President Emmanuel Macron was re-elected on April 24.
According to the Elysée, Borne was “in charge of forming a government”.
She dedicated her candidacy to “every little girl” in a ceremony celebrating the takeover of power on Monday.
“Follow your dreams all the way,” Borne said in a speech. “Nothing should hinder the struggle for women’s place in society.”
Earlier on Monday, the Elysée Palace announced that Castex had submitted his resignation.
Castex, the former mayor of his small hometown of Prades in southwestern France, was a little-known when he became Prime Minister in July 2020. He continued to lead France’s response to the Covid-19 pandemic.
