McClaren has been appointed as Erik ten Hag’s new assistant coach at Manchester United alongside Mitchell van der Gaag.
Ten Hag, who took over from Ralf Rangnick as Man United boss on Monday, worked with McClaren as his assistant at FC Twente in the Dutch Eredivisie.
The 61-year-old’s last coaching role was at Championship side QPR during the 2018-19 campaign but now he returns to Manchester United for the first time since 2001. McClaren spent two years as Sir Alex Ferguson’s assistant manager during a successful period at Old Trafford.
He left to become Middlesbrough manager during the club's most successful period between 2001 and 2006 as he led the Teesside outfit to a League Cup win in 2003-04 and UEFA Cup final following a memorable run in 2005-06.
McClaren left Boro to replace Sven-Goran Eriksson as England manager in 2006 but he failed to qualify for Euro 2008 with the Three Lions.
McClaren, Ten Hag and van der Gaag were all present at Selhurst Park as The Red Devils suffered a 1-0 final day defeat to Crystal Palace as they ended the Premier League season in sixth place, just nine points ahead of Newcastle.
"I thought I worked hard until I met Erik," McClaren said prior to his official appointment.
"His great strength lies not just in his attention to detail and organisation. He has a clear philosophy of how he wants to play football, the environment he wants to create."