Root, 24, beat the record of David Gower, who was 26 when he racked up three figures against Sri Lanka in the 1983 World Cup in Taunton, as he clubbed 121 from 108 balls in England???s score of 309-6.
But majestic centuries from Lahiru Thirmanne (139no) and Kumar Sangakkara (117no) helped Sri Lanka knock off their target of 310 in 47.2 overs and consign Eoin Morgan???s men to a third defeat in four World Cup matches.
Morgan won the toss at Westpac Stadium and opted to bat first, just as he did last week in the eight-wicket mauling by co-hosts New Zealand at the same venue.
England started briskly, though, racing to 62 without loss from 9.1 overs with Ian Bell finding the fence regularly, before Moeen Ali chipped Sri Lanka skipper Angelo Mathews tamely to mid-off.
Gary Ballance (6) and Bell (49 from 54) then went before the end of the 21st over as England were pegged back, but Root???s partnerships of 61 with skipper Eoin Morgan (27) and 98 with James Taylor (25) gave the three-time runner???s up real momentum.
Root thrashed 14 fours and two sixes en route to his highest ODI score, profiting from being dropped at the start of his innings by Mahela Jayawardene in the slip cordon.
Jos Buttler (39 from 19) took over the mantle once Root had been given out lbw to Rangana Herath, crunching six fours and a six, despite being hit on the side of the grille by a Lasith Malinga bouncer.
England would have headed into the interval in buoyant mood but Root dropped Thirimanne on two and things got steadily worse for Morgan???s men, who headed into the match unchanged.
Thirimanne and Tilakaratne Dilshan (44 off 55) put on exactly 100 for the first wicket, before Dilshan, who scored an ODI best of 161 not out against Bangladesh in midweek, hit Moeen lazily to Morgan at mid-wicket.
However, Thirimanne and his fellow left-hander Sangakkara were at their magnificent best thereafter, snuffing out any hope of an England win with some brutal strokeplay.
Thirimanne and Sangakkara put on an unbroken stand of 212 for the second wicket, with the former becoming the youngest Sri Lankan to score a World Cup hundred and the latter racking up a 23rd ODI ton just weeks before he retires from the format.
Thirimanne creamed 13 boundaries and two maximums ??? including the match-settling one ??? in his knock but was outgunned by Sangakkara, whose 86-ball innings handed him a strike rate of 136.05.
James Anderson, Stuart Broad and Steven Finn went at over six runs per over, with Chris Woakes was smashed for in excess of seven as Sri Lanka wrapped up their triumph with 16 balls to spare.
England sit on two points in Pool A and must now beat Bangladesh and Afghanistan in their last two group-phase encounters to stand a chance of making the quarter-finals.