, the former New Zealand captain, has been confirmed by the ECB as England Men's new Test head coach, on a four-year deal.
McCullum, who will step down from his role as Kolkata Knight Riders' head coach at the end of the ongoing IPL, played 101 Tests for New Zealand between 2004 and 2016, scoring 6453 runs at 38.64 with a highest score of 302 against India in 2014, which remains the only triple-century made by a New Zealander.
Although he has never before taken charge of a red-ball team in his coaching career, McCullum was New Zealand captain for 31 Tests from 2013 until his retirement, earning plaudits for a positive approach which was also instrumental in the white-ball team's run to the World Cup final in 2015. In 2020, he also coached Trinbago Knight Riders to the Caribbean Premier League title.
His exact arrival date in the UK is dependent on KKR's qualification for the IPL play-offs, although with two group-stage games remaining - the last of which comes against Lucknow Super Giants on May 18, they are an outside bet to reach the competition's final four.
Work permits permitting, he is expected to link up with the squad - which will be led by England's newly-appointed captain Ben Stokes
- ahead of the first Test against New Zealand at Lord's, starting June 2, and will now have an input in next week's selection meeting too.
Rob Key, England's director of men's cricket, acknowledged that McCullum's appointment was a calculated gamble, adding that it was "time for us all to buckle up and get ready for the ride", but praised his proven ability to change team cultures for the better - an attribute sorely needed after a grim 12 months for England's Test team, in which they have won one of their last 17 Tests and sunk to their lowest ICC ranking since 1995.
"I'd like to say how pleased I am to be given this opportunity to positively contribute to England's Test cricket set-up and move the team forward into a more successful era," McCullum said in an ECB press release.
"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 had initially been earmarked for England's white-ball coaching role, not least because of his close personal and professional relationship with the captain Eoin Morgan
- he was master of ceremonies at Morgan's wedding in 2018 and last year the pair guided KKR to the IPL final.
However, once it became clear he was interested in the Test job, McCullum emerged as the unanimous choice of an ECB selection panel comprising Key, CEO Tom Harrison, performance director Mo Bobat, and Andrew Strauss, who is a strategic advisor to the board having stood down from his interim MD role in the wake of Key's appointment.
In particular, it was felt that his vision for the Test team was in alignment with that of the new captain Stokes, who last week called for "selfless" cricketers to help revive the team's fortunes, after being unveiled as the successor to Joe Root, who stood down last month after a record 64 Tests in charge.
"We are delighted to confirm Brendon as England Men's Test head coach," Key said. "It has been a real privilege to get to know him and understand his views and vision for the game. 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.
"I'd like to thank Tom Harrison and the ECB Board for their support in this process. 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."
Venky Mysore, CEO of the Knight Riders franchises, joked that the ECB were taking all of KKR's successful coaches, having earlier seen Trevor Bayliss move into the head coach role from the IPL. Mysore, who recruited McCullum as head coach at the two Knight Riders franchises in IPL and CPL, said the former New Zealand captain's biggest strength was being transparent and positive.
"First of all he is a good human being. It starts with that," Mysore said. "A national coaching role is a little different because you have so much more time with work with someone around the year. But it starts with being a good human being and then being able to connect with people and build relationships. When you do that and earn their trust that's when people are open to listening to you.
"At the core of it, you've got to be a trustworthy person who people take a liking to. It takes time. Then they are open to listening to what you are saying. That is the key. It seems simple but it is such a hard thing to get. Ultimately you need to have that core capability, which is to be able to be transparent, be honest and build those relationships. And that's his strength. He has got a very positive vibe about him, always."
England's white-ball team is not in action until their three-match ODI series against the Netherlands which starts on June 17, and so there is less urgency in filling the second of England's coaching vacancies.
Candidates for that role include Gary Kirsten - who had been among the frontrunners for the Test role - alongside the Australia women's coach Matthew Mott and Paul Collingwood, who filled the interim coaching role for both the red- and white-ball legs of England's recent tour of the Caribbean.
Andrew Miller is UK editor of ESPNcricinfo. @miller_cricket