In his first game back since taking bereavement leave after the death of his father in Zimbabwe, de Grandhomme plundered an unbeaten 74 off only 40 balls, with five sixes and seven fours to make a potentially tricky chase into a doddle.
He put on 109 with Henry Nicholls to rip the match away from Pakistan, enable New Zealand to make 263 for five, to win by five wickets, take a 4-0 lead in the five-game series, and ensure the Black Caps would have their best run of victories across all forms in its history.
New Zealand began their chase for 263 well with an 88-run stand between openers Colin Munro and Martin Guptill.
But legspinner Shadab Khan sliced through the innings as New Zealand lost their way, and four for 11 in 35 balls.
Captain Kane Williamson and Henry Nicholls steadied the innings before de Grandhomme ripped into his work.
When he arrived New Zealand needed 109 off the last 15 overs.
Victory was achieved with 4.1 overs to spare.
De Grandhomme's first ODI half century came off 25 balls and no Pakistan bowler was spared.
He showed superb timing and what shaped as a testing late chase turned into a far easier proposition.
At one point he needed 10 off two balls to pip Martin Guptill's 17-ball effort against Sri Lanka at Christchurch in 2015 as New Zealand's fastest.
Two partnerships of substance ensured Pakistan's bowlers would have something to work with at the halfway point.
Add in 22 clubbed off the 50th over from Trent Boult by veteran Mohammed Hafeez and Pakistan fought their way to 262 for eight, having won the toss.
Hafeez was run out by Boult off the final ball of the innings, having got to 81 off 79 balls, taking three sixes off that last over.
Pakistan had to dig their way out of a hole, not for the first time in the series. But they had stands of 86 and 98 for the third and sixth wickets.
Even allowing for Hafeez's late hitting, the total was still on the low side for the ground, and in this form.
Captain Kane Williamson took two for 32 off 10 overs and seamer Tim Southee took three wickets.
Meanwhile, Pakistan allrounder Shoaib Malik was being monitored for showing signs of concussion after being struck by a return.
Malik was hit by a sidearm throw from Munro when the batsman returned to his crease after attempting a quick single.
The ball struck Malik's uncovered head and he was treated on the ground by the Pakistan team physio Vib Singh and the match doctor.
He did not return to field in New Zealand's innings.
"He did not have any symptoms of a concussion and was able to carry on playing,'' Singh said.
"After getting out he was reassessed and was showing delayed concussion signs. He is doing well and is resting.''
Malik didn't take any further part in the match.
