The former New Zealand captain, who is currently in charge of the Kolkata Knight Riders, will leave his role with the Indian Premier League side and focus on his new position as he looks to lead England out of their current rut.
Following a disastrous run of just one win in 17 matches, England have slipped to last place in the World Test Championship. What's more, the side were swept aside by Australia in their recent Ashes series.
While McCullum may lack experience - having never coached at first-class or Test level - the 40-year-old is regarded as one of the most progressive thinkers and dynamic leaders in the sport.
During his time as New Zealand captain, McCullum set the team on a path to their current golden era - one which has seen the Kiwis claim the inaugural World Test Championship and reached two World Cup finals.
Rob Key, who oversaw the appointment process alongside ECB chief executive Tom Harrison, strategic adviser Sir Andrew Strauss and performance director Mo Bobat, revealed his excitement about the prospect of McCullum linking up with new England captain Ben Stokes.
He said: "We are delighted to confirm Brendon as England men's Test head coach. It has been a real privilege to get to know him and understand his views and vision for the game.
"I believe in Brendon and Ben Stokes - a formidable coach and captain partnership. Time for us all to buckle up and get ready for the ride. I believe his appointment will be good for England's Test team.
"He has a recent history of changing cricket culture and environments for the better, and I believe he is the person to do that for England's red-ball cricket. We were incredibly fortunate to have a seriously strong list of quality candidates for the post, with Brendon demonstrating he was our number one choice."
McCullum retired from international cricket in 2016 with 101 Test caps under his belt. The new coach revealed he has firm views about taking England out of their current slump.
He said: "In taking this role on, I am acutely aware of the significant challenges the team faces at present, and I strongly believe in my ability to help the team emerge as a stronger force once we've confronted them head-on.
"I've enjoyed several robust conversations with Rob Key about the direction of travel for the team and have found his enthusiasm contagious. I'm no stranger to bringing about change within a team environment, and I can't wait to get started.
"Ben Stokes is the perfect character to inspire change around him, and I look forward to working closely with him to build a successful unit around us."
McCullum is expected to arrive in the UK in time for the upcoming three-match Test series against New Zealand, subject to pending visa approvals.
Meanwhile, England turn their attention to appointing a new white-ball head coach, with Paul Collingwood the current favourite.
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