On Monday, the Magpies confirmed that a compensation package had been agreed with Man United to release Ashworth, who had been on gardening leave for four months, from his contract at St James' Park.
Newcastle were initially keen to appoint Crystal Palace's sporting director Dougie Freedman as Ashworth's replacement, but he opted to turn down their approach in May.
Forty-two-year-old Mitchell was another name who featured highly on their list of targets and following a lengthy interview process, his appointment at Newcastle has now been confirmed.
Newcastle eyeing 'sustainable success' under Mitchell
"It's with immense pride that I accept the responsibility of being Newcastle United's new sporting director," Mitchell told the club's official website.
"I've seen the recent growth and ambition of the club. This, plus the amazing fan base, made the decision to join an easy one.
"I can't wait to get started and help continue the organisation's growth and long-term competitiveness in all areas of elite football performance."
Newcastle CEO Darren Eales added: "We are delighted to welcome Paul to Newcastle United. He brings a successful track record and wealth of experience to the club.
"His extensive and diverse experience in football leadership, cultural change and recruitment mean he is the ideal person to lead our sporting directorate.
"We look forward to working with Paul and supporting him in our pursuit of long-term, sustainable success."
Mitchell's eye-catching CV admired by Newcastle
Mitchell represented the likes of Wigan Athletic and MK Dons during his playing days before ending his career early due to injury, and he has since established himself as "a well-respected sporting executive with a track record of success" in the eyes of Newcastle.
After a spell as MK Dons' first-ever head of recruitment, Mitchell moved to Southampton where he oversaw a number of shrewd acquisitions including Sadio Mane, Dusan Tadic, Graziano Pelle and Steven Davis following the club's promotion to the Premier League.
Mitchell further cemented his reputation during a successful spell as head of recruitment at Tottenham Hotspur - following Mauricio Pochettino to North London - where he played an integral role in signing players such as Son Heung-min, Dele Alli, Toby Alderweireld and Kieran Trippier.
He has also had success abroad, firstly as head of recruitment at Bundesliga club RB Leipzig, before moving to Ligue 1 outfit Monaco in 2020 to become their sporting director, the role he will now take a Newcastle.
One of Mitchell's first tasks will be to get to grips with the summer transfer window, with Eddie Howe's side ready to spend off the back of their PSR compliance at the end of last month.
Newcastle have already completed four signings this summer, bringing in Lloyd Kelly, Odysseas Vlachodimos, John Ruddy and Lewis Hall, the latter has made his loan move permanent.