Borne formerly served as minister of the environment, transportation and labor.
She will lead a French government likely to be tasked with delivering on President Macron’s election campaign priorities: retirement reform and the scaling up of policies designed to combat climate change.
She dedicated her nomination to “every little girl,” during a ceremony on Monday marking the transfer of power.
“Follow your dreams all the way,” Borne said in a speech. “Nothing must hold back the fight for the place of women in society.”
Earlier Monday the Elysée Palace announced that Castex had tendered his resignation.
The former mayor of his small hometown of Prades in southwest France, Castex 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.