Hope Hicks, the former communications director for the White House, is joining Rupert Murdoch’s media empire.
Ms Hicks, 29, will serve as chief communications officer for Fox, the parent company of Fox News and other news outlets.
She left the White House in April, after rising to prominence as a close Trump adviser during the 2016 presidential campaign.
Fox News is President Donald Trump’s favourite cable channel.
This summer he named former Fox News co-president Bill Shine as assistant and deputy chief of staff for communications.
Mr Shine resigned from Fox News last year as the network’s handling of sexual misconduct scandals came under scrutiny, though Mr Shine himself was not accused of any harassment.
Mr Trump is also reported to have a close relationship with host Sean Hannity, a leading cable news cheerleader of the president.
Ms Hicks had worked for the Trump Organization since 2014.
She kept a low profile while at the White House but was said to be one of Mr Trump’s most trusted confidantes.
The announcement that she will take a role at Fox is the latest in a series of appointments 21st Century Fox has been making in anticipation of a major reorganisation.
The firm is preparing to sell a big portion of the existing company to Walt Disney Co and spin off its news assets, including Fox News, into a new company called Fox.
Those deals are expected to be completed in the first half of 2019, pending regulatory approval.