Root's masterly 166 off 139 balls helped England, who won the first ODI by 238 runs at Edgbaston, chase down a target of 309 at Sophia Gardens in Cardiff.
After winning the toss and electing to bowl, England made an early breakthrough to make it 6-1 by claiming the wicket of 18-year-old opener Jewel Andrew (0), before Brandon King and Keacy Carty put on a run-a-ball 141-run stand to calm West Indies' nerves.
Spinner Adil Rashid broke the partnership when King (59) holed out to long-off, and all-rounder Will Jacks struck to end Carty's fluent knock of 103 off 105 balls, reducing West Indies to 3-205.
Carty was dropped twice — on one and 41 — and also survived a few run-out chances, before going on to punish the England bowlers by notching his fourth ODI century.
But Rashid continued to shine with the ball, trapping Shimron Hetmyer (4) lbw and finishing with figures of 4-63 to become England's highest wicket-taking spinner across formats.
West Indies lost wickets at regular intervals, but captain and wicketkeeper-batter Shai Hope (78) and Justin Greaves (22) came to their rescue, helping the visitors to a total of 308 all out in 47.4 overs.
England opened their run chase in disastrous fashion, reduced to 2-2 after both openers, Jamie Smith and Ben Duckett, were dismissed for ducks.
Dropped on 30 by Hope off the bowling of Jayden Seales, England skipper Harry Brook went on to make 47 as he tried to stabilise their innings before falling to Alzarri Joseph, who ended with figures of 4-31.
With former captain Jos Buttler out for a duck and Jacob Bethell trapped lbw, Root anchored the innings, putting together a 143-run partnership with Jacks (49).
But after Brydon Carse (2) was dismissed, Rashid (10) provided Root help on the other end as England chased down the target with seven balls to spare.
"It's a sign you're getting old, having played as many games as that," the 34-year-old said with a smile after becoming the first Englishman to go past 7,000 ODI runs.
"I want to give as much as I can for England for as long as I can.
"I've still got a lot to give and a lot more runs in there and hopefully, an innings like that shows that."
The final ODI will be played at The Oval in London on Tuesday.
