The Kansas City Royals claimed their first MLB World Series in 30 years after a comeback 7-2 win over the New York Mets.

Trailing 2-0 heading into the ninth inning at Citi Field on Sunday, the Royals appeared set for a loss in game five.

But they rallied with two runs before scoring five more in the 12th inning to seal a 4-1 series victory.

Eric Hosmer had a crucial double before Lucas Duda's bad throw from first base to home saw him score on a Salvador Perez groundout in the ninth.

Christian Colon hit a go-ahead single in the 12th before Alcides Escobar's RBI double and Lorenzo Cain's bases-clearing double.

Mets pitcher Matt Harvey was outstanding for eight innings as he shutout the Royals before urging manager Terry Collins to send him out again.

It was a decision that would prove costly as Harvey walked Cain, who was brought home by Hosmer.

Jeurys Familia replaced Harvey but Duda's poor throw would see the Royals level the game.

Kansas City went ahead in the 12th and added runs to clinch their second championship and first since 1985.

Royals starter Edinson Volquez was solid, but he gave up the two runs as part of his six innings.

Earlier, Curtis Granderson blasted a lead-off home run to right-centre field to give the Mets a 1-0 lead that was doubled in the sixth.

Duda's sacrifice fly was enough to bring in Granderson to score, but the Royals would come back once more.