England's victory target was revised to 308 off 42 overs due to the weather having seen the tourists show real guile and style in their innings.
Mendis smashed 77 from 62 balls with 13 boundaries in an entertaining innings while Gunathilake hit 62 with seven fours.
Chandimal weighed in with a rapid 63 from 51 balls including three sixes while Matthews made Mendis and Gunathilake put on 128 from 111 deliveries.
Sri Lanka's batsmen burst out of the blocks, batting with confidence and a real positive attitude.
Kusal Perera was superbly run out by a direct hit from Jonny Bairstow's throw.Mendis picked up from his half century in Bristol to go on the attack and struck seven boundaries in 11 fours to stun the the England attack.
Gunathilake joined the fun as Sri Lanka surged to 100 from just 91 balls, defying the blustery, cloudy and drizzly conditions.
Mendis lit up proceedings with three boundaries came off one Willey over and must have made England captain Eoin Morgan wonder if he had made the right choice to insert the tourists on a flat Oval wicket despite the overcast conditions.
England opener Alex Hales returned to the dressing room for treatment on a back injury sustained while trying to stop a four on the damp slippery outfield.
Rain forced the players from off at 2.19pm with Sri Lanka well set on 127-1 with Mendis unbeaten on 75 and Gunathilake 48 not out.
Sri Lanka lost two quick wickets following a two and a quarter hour rain delay which reduced the game to a 42-over aside contest. Rashid grabbed both of them in 11 balls.
Angelo Matthews hammered Ali into the crowd and Chandimal did the same to Plunkett twice. The second six, a deliberate uppercut over third man with both feet off the ground, took him to 50.
Candimal was bowled around his legs by Willey but Matthews ensured the tourists passed the 300 total.
Hales was unable to open the England batting when the home side came out to try and chase down the winning total.
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