The news of his move was formally confirmed in Spa on Friday, and he will be able to start in his new job at the end of this year.
“On behalf of all at Williams Racing, I’m delighted to be welcoming Pat," team boss James Vowles said in a statement.
"His knowledge and experience will further strengthen the team's technical capabilities and pursuit of excellence as we build the next chapter of Williams.
"Pat has been a core part of winning teams throughout his career, he is one of the most respected experts in our industry and I’m excited to begin work with him when he joins in November.”
Williams has been without a technical leader since the departure of Francois-Xavier Demaison in December as part of the management shake-up that also saw team principal Jost Capito leave.
New team principal Vowles made it clear from the off that finding a new technical chief as well as a head of aerodynamics was a key priority, although he stressed that he would take his time and wait for the right candidate to be available and free of gardening leave.
Vowles also made it clear that the team’s improved form and financial health is proving attractive to potential employees.
“I think what I can tell you is that as people's heads are held high here, and other people's heads are turned towards us as well,” he told Motorsport.com recently.
“I think everyone's realised, actually that the investment is real, the direction of travel is real.
“I've been speaking out publicly about it in F1 Commissions, I have money, let me spend it so this becomes a meritocracy. But that's attracting people that realise we're on the pathway to moving forward.”
The 59-year-old Fry currently serves as chief technical officer of Alpine, having joined the Enstone outfit for a second stint for the start of the 2020 season.
He began his F1 career with the then-Benetton team in 1987, working on active suspension and latterly serving as a race engineer.
He joined McLaren in 1993 when Ayrton Senna was still at the Woking outfit. He worked on active suspension before it was banned and he again moved into a race engineering role, before latterly becoming chief engineer in alternate years for the 2005, 2007 and 2009 cars.
He moved to Ferrari in 2010 as assistant technical director, before working his way through a variety of roles, including head of trackside engineering and director of chassis.
However, he left the team at the end of the team’s difficult 2014 season, the first run to the hybrid regulations.
Fry spent the 2016 season as an engineering consultant for Manor Racing before returning to McLaren for a brief stint in what was always acknowledged as a temporary arrangement.
His return to Enstone and the then-Renault team was announced in November 2019. It’s understood that he is still working at Alpine for the time being, however his departure will mean further changes to the management structure as the team continues to undergo a shake-up.