In becoming the first New Zealander to join the elite club of cricketers who have scored a test triple century, he saved the test match and secured a series win. For McCullum, the result was more important than the personal milestone.
"After day two we discussed it as a group," he told Campbell Live afterwards. "We'd had such a brilliant summer on the back of a pretty good summer [against England] last year. With all the goodwill and all the rapport building up with the rest of the country who love this game we didn't want that to be undone ... at the last hurdle."
It was an innings that stopped a nation - children wagged school with their teachers' blessing, crowds gathered around high street appliance stores and office TVs, fans queued for a seat at the Basin Reserve.
Not bad for test cricket, a five-day game threatened by waning interest outside the three nations where it is akin to religion - England, Australia and India.
But fans of our summer game are fickle, and the players know that if their small nation can't foot it with cricket's powerhouses we risk becoming a cricketing backwater. In recent years we've slipped down the rankings in all three forms of the game - tests, one-dayers (ODIs) and Twenty20 - and McCullum has been one of the whipping boys.
He is variously too impetuous, too ambitious, too impatient, too selfish, one who picks and chooses when and what role he plays ...
It's a sport with its share of internal controversies and his name has been linked to a few, often with scant evidence.
He was our finest wicketkeeper/batsman until injuries forced him to give up the test keeping gloves in 2010. His instinctive batting has won and saved matches for his country in all three formats but he has battled inconsistency - and critics have seized on the fallow periods between the highs. Even as this summer started, some questioned whether he merited his place for his batting alone.
A big factor is perception - the public think they know the man through his on-field actions, viewed through a television screen.
It's the big hits and improvised shots in the short formats that stick in the mind; we forget the many responsible test innings he's played.
"I've never known an athlete who is so different to the public perception of him," said one sports administrator.
His on-field aggression and risk-taking are sometimes mistaken for arrogance, says Sky Sport commentator Craig Cumming, a former teammate and close friend of McCullum. "His personality comes out on the field but what it is is confidence, and self-belief. He'll gamble but that's just being positive - it's 'how do I win a game?"'
Off the field, those who know him describe a quality person and father, sincere and caring "with a soft side", a generous friend eager to give his time. He likes a beer and a wager on the golf course and has minor shares in race horses (gallopers and pacers). He has business interests with former Black Caps captain Stephen Fleming and bowler Geoff Allott, and the hugely lucrative Indian Premier League (the Twenty20 franchise competition) might have been invented for him.
But McCullum likes to play up his working class roots. Asked what emotions he felt after reaching the milestone 300 on Tuesday, he told the press conference: "Nah, no tear in the eye - I'm from South Dunedin."
He's a "cricket, racing and beer man" who comes across in post-match interviews as personable and grounded, conscious of the players' obligations to the fans and the high expectations.
Born into a sporting family (father Stu was a swashbuckling Otago batsman) he was a freakishly talented athlete who kept Dan Carter out of the South Island rugby secondary schools side in his sixth form year.
He would play for Kings High School's first XI soccer team and the rugby first XV on the same day. The "boys from the flats" took most satisfaction showing the more privileged kids from the hills - Otago Boys High School - who ruled.
His old coach, John Cushen, remembers McCullum making the first XI cricket team when he was "about the height of the stumps" in the third form. Later, as captain, he proved a charismatic and inclusive leader - qualities now being brought to bear with the national side.
"He would always say 'we' rather than 'I' - he was very tough but very loyal," Cushen said. "And he's always got time for those who helped him up the ladder."
McCullum was rushed into the Black Caps one-day side in 2002, aged 20, and met wife Ellissa on his first tour to Australia. They have two children, Riley, 9, and Maya, 3, and their third child is due in May. Home is in Christchurch.
His family is understood to be hugely supportive but cricket is now a year-round treadmill and with his chronic back and knee problems, McCullum is on borrowed time. He has two protruding discs pushing on his spinal cord and arthritis in his back, and has had numerous knee operations.
He was on painkillers in Bangladesh in October and returned home with "dark thoughts" about the future.
"That's not how the game is meant to be played," he told the Herald's David Leggat. "I've got a 60-year-old back and if I want to be able to lift my kids at 35 it's something I've got to entertain."
Yet when he gave up the test wicketkeeping gloves in 2010, critics said he was saving himself for cricket's more lucrative short forms, particularly the IPL. Only when he stopped keeping in one-dayers did the public accept his injuries were career-threatening.
"That's one of the things that's hurt him most - that people think he's selfish," Cumming says. "He has a huge amount of pride for his country."
Then there was the intrigue over the captaincy. As heir-apparent under Daniel Vettori, he was stripped of the vice-captaincy in mid-2009 amid apparent concern that he was not an ideal role model for younger players - while a beer and a smoke never affected his performance, it might send the wrong message.
A year later, McCullum was thrust into a protracted leadership contest with Ross Taylor and lost.
"... in the end they decided to stay with the more typically New Zealand, safer approach," he told the Herald's Dylan Cleaver.
But he responded with his bat - three centuries in 2010, including a double in Hyderabad to save a test against India. He moved up to open the innings for the good of the team - there was a dearth of candidates for cricket's most technically demanding role.
But 16 months later, Taylor was dumped as captain days after scoring a century to clinch a rare away win in Sri Lanka - an appallingly-timed call by new coach and chief selector Mike Hesson.
New captain McCullum's close connection to Hesson, his former Otago coach, fuelled more conspiracy theories. Public sympathy for Taylor and wider frustration with the team extended to the retired players' network and McCullum was falsely accused of being "in" on the coup. It took a threat of legal action to clear his name.
Now 16 months on, it seems that Hesson's call has come up trumps. Taylor, our best batsman, had a sensational summer. McCullum's captaincy has proved inspired; he and Hesson are credited with building a culture of inclusiveness and self-belief which is bringing results.
When McCullum strode on to the Basin Reserve on Sunday morning, New Zealand were 52 for 3 in their second innings, still nearly 200 behind India's first innings score and nearly three days from safety. Soon it was 94 for 5 and a summer which had loomed as a turning point for New Zealand cricket looked like another false dawn.
"I think the driving factor in the innings he played was fear of losing, as the captain of a team which had performed exceptionally well all year," says Cumming. "I don't think he was worried about personal milestones at any stage until his team was in a safe position."
McCullum's test batting average before this summer was an acceptable mid-30s but, with a century, double-century and now the triple in six tests, his record is more reflective of his talent. As captain, he's averaging 50.
"The great thing now is it's written down on paper," says Cumming. "Brendon has always been world-class but until now it wasn't recognised in the statistics. I can't think of a better person for that to happen to."
