Buttler previously led England during a series win in Bangladesh in October 2016 when Morgan refused to tour because of security concerns.
The wicket-keeper found himself unexpectedly thrust into the role when regular one-day global skipperEoin Morgan pulled out shortly before the start of Saturday's second one-day worldwide in Cardiff.
Australia were bowled out for 304, with seamer Liam Plunkett taking 4-53.
- "Key wicket" - But an Australia side missing star batsmen Steve Smith and David Warner, both given year-long bans for their roles in the Cape Town ball-tampering fiasco in March, just fell short.
However, Plunkett and spinner Adil Rashid (3-70) excelled in the final over to dismiss Marsh and run through the Australia tail. We couldn't quite finish it off at the end there.
"I was pleased with my hundred".
"I don't think it's impossible to do so", said Buttler.
"Obviously we would have liked to have restricted England to less than 340-odd but the way Marsh played throughout our innings showed we could have chased them if we'd had someone to go with him before Agar", said Australia captain Tim Paine.
"It's just little moments like that where someone just has to put his hand up and say, "Right, I am going to be the man that's going to get through here and do what my team needs me to do".
The defeat means the world champions have now won only two of their last 15 ODIs on a day where England's powerful batting line-up dominated an inexperienced Australiaattack.
Rain breaks when he was 66 not out and unbeaten on 91 failed to disturb Roy, who drove Marcus Stoinis and pulled Andrew Tye for sixes during a 97-ball century that also included nine fours.
AJ Tye brought Roy's innings to a close when a blood-stained Paine took a superb diving catch, having being hit in the face minutes earlier when a slower delivery from the West Australian bounced off the turf and struck him on the nose.
Jonny Bairstow and Roy got the hosts off to a buccaneering start before the in-form opener departed for 42 when Kane Richardson found the edge of his bat and Paine took a low catch.
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