Borne previously served as minister of the environment, transport and labor.
He will lead a French government that will likely be tasked with delivering on President Macron’s election campaign priorities: retirement reform and expanding policies designed to combat climate change.
Borne has been “commissioned to form a government,” according to the Elysee.
She dedicated her nomination to “all the little girls” during a ceremony Monday marking the transfer of power.
“Follow your dreams to the end,” Borne said in a speech. “Nothing should stop the fight for women’s place in society.”
Earlier Monday, the Elysee Palace announced that Castex had tendered his resignation.
Castex, a former mayor of his small hometown of Prades in southwestern France, was a little-known figure when he became prime minister in July 2020. He went on to lead France’s response to the Covid-19 pandemic.