The exact value of the agreement was not revealed when it was formally announced in Abu Dhabi but reports say it will earn the 23-year-old anywhere from £62m in a five-year contract to as much as £156m over 10 years.
The larger figure would dwarf Tiger Woods' £124m 10-year deal with Nike.
But his new-found wealth will not change him, according to friends who knew him at a time when every penny counted.
"He has all the money he needs, but it's not the money any more. It's about winning majors and staying the world's No1," said his uncle Colm.
"He'll just be the same Rory - up for the craic, a joke and the banter".
Just a few years ago, Rory's parents, who still live in Holywood, Co Down, worked every hour they could to pay for his golf.
His father Gerry worked as a barman and swept the changing room floors of a nearby sports club to make ends meet.
His mother Rosie worked night shifts at a factory making masking tape.
The Northern Ireland star, whose girlfriend is Danish tennis player Caroline Wozniacki, has become hot property after winning two of golf's major tournaments in two years; the US Open and the US PGA.
But no matter what his bank balance, he will always be the same, according to Stephen Crooks, 23, head professional at Holywood.
"The first four or five years on the tour hasn't changed him - the money certainly won't," he said.
"He is pretty much set in his ways, a contented, genuine guy who just wants to play golf and win.
"We haven't seen him around here since July and I'd imagine he'll only pop in to see us when he is over for the Irish Open and the British Open in the summer.
"That's just the way it is. Rory has moved on. The last time he was in Holywood he helped raise £60,000 for the club, which is into the next stage of major refurbishment, including a new professional shop for myself.
"That's the sort of impact he has made. It happens everywhere he goes."
Home now is a luxury pad he bought before Christmas at West Palm Beach, Florida, which he shares with Ms Wozniacki.
He had a huge house in the hills above Belfast, complete with practice area, but that is still up for sale.
By becoming the face of Nike he is expected to boost the sportswear firm's sales of clubs, balls and clothing in the US market, where he has a large fanbase.
McIlroy reportedly earned more than $13m (£8m) in prizes last season, with a further $12-15m (£7.4m-£9.3m) in endorsements and appearance payments.
He is currently number one in the world golf rankings, with Woods in second.