Since adding the white-ball position to his Test role in January 2025, McCullum had overseen 10 ODI losses in a row on England's travels.
The tourists knew a second defeat in Colombo in three days on Saturday would ensure they had won only one of their last eight 50-over series, but they produced a strong all-round display to make it 1-1 ahead of Tuesday's decider.
Captain Harry Brook deployed 41 overs of spin in an impressive bowling performance to bowl Sri Lanka out for 219 before Root made an imperious 75 on a slow wicket under lights to lead England home.
Following a dismal Ashes tour which culminated in a 4-1 defeat, this was a morale-boosting win which helps limit the damage to England's world ranking - as pressure builds on their automatic qualification for the 2027 World Cup.
England played Sri Lanka at their own game by using six spinners, who claimed a combined 7-180.
Sri Lanka's innings was sedate, strangled by England's spinners as Adil Rashid and Liam Dawson again starred.
Skipper Charith Asalanka top-scored with 45 but the hosts never gained momentum before being bowled out in the final over.
Having fallen 19 short of their target of 272 at this ground on Thursday, this time England made no mistake in their chase.
They'd sprung a surprise by naming 21-year-old spin-bowling allrounder Rehan Ahmed as opener for the first time in international cricket - replacing the injured Zak Crawley.
He was bowled by Dhananjaya de Silva for 13 - aided by four overthrows - as the gamble failed to pay off and England's chase started in wobbly fashion.
But Root caressed his first ball for four while Ben Duckett took the attack to Sri Lanka's spinners as the pair raced to a stand of 69 in 65 balls.
When Jeffrey Vandersay produced a ripper to bowl Duckett for 39, there would have been tension in the England camp at 2-88, with 132 still required, and when Jacob Bethell slapped de Silva to cover for just six, panic might have set in.
But Root marched on, reaching his 45th ODI fifty to steer England to 3-114 at halfway and he was joined in a composed 81-run stand by Brook.
Asitha Fernando had Root trapped lbw to make it 4-178, the 35-year-old furious when DRS upheld the decision despite him believing he had edged it.
Brook then missed a sweep off Vandersay to depart for 42 as the nerves jangled again, but Jos Buttler crashed 33 to secure a first away win since November 2024.
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