The 29-year-old emerged from his brother's shadow at the World Twenty/20 Cup with some eye-catching performances.
Usually, it is younger brother Brendon McCullum who is in the limelight, but Nathan's frugal off-spin and explosive batting attracted the attention of English county side Lancashire.
Lancashire eventually opted for Pakistan all-rounder Shoaib Malik ahead of McCullum, but when Malik became unavailable late in the piece, McCullum got the call up.
"It is going to be an amazing opportunity for me," McCullum told the Otago Daily Times.
"This is a way, how shall I put it, of being my own person and it is another opportunity to further my career playing twenty/20 in England. Not a lot of people get that opportunity."
Asked if it was a lucrative opportunity, McCullum responded by saying twenty/20 could be fruitful "for some people".
"I guess it depends on your brand and whether they like the style of cricket you play.
I enjoy it but you just never know what is going to happen.
"For me it is about me trying to get a New Zealand contract for a start.
That's my first goal."
With the amount of money on offer in the various twenty/20 tournaments, players' loyalties have been, and will continue to be, tested.
It has been a topic of discussion for the McCullum brothers.
"We've talked about it recently. We're Otago players and we always will be. It is something I'm very passionate about.
"I love playing for Otago and I love playing with my mates. It is how I've got to where I am today and I'll never forget that."
McCullum joins Australian Simon Katich as one of the two overseas professionals at the club and will not be just "one of the boys" any more.
As one of the stars he will be expected to perform.
"You want to have that pressure, you want to have that expectation. It is about going and doing what you do and trusting that it is going to be good enough to make you succeed."
McCullum was yesterday eagerly awaiting the arrival of his visa.
The tournament got under way earlier this month and Lancashire opened its campaign with a 23-run win against Durham.
But while McCullum was eager to join his new team, he was also torn.
His partner, Vanessa Waldron, is expecting the couple's first child in August and, with McCullum away so often, she has become a bit of a cricketing widow.
"It has been pretty tough for her," he acknowledged.
"I've been away a lot this year," adding they had recently moved into a new home.
"She has pretty much set up shop on her own."
McCullum expects to return to New Zealand on July 20 and hopes to get selected for the tour of Sri Lanka in August-September.
That leaves a narrow window for the baby's arrival.
