Usman Khawaja, stumped on 89, led Australia to chasing down Sri Lanka's 239-8 with five wickets and five overs to spare in Southampton.
Khawaja gave Australia an injury scare when he was struck on the left knee by the ball while fielding. He limped off the Rose Bowl and Australia later said it was only a contact injury and he would bat.
It was his second injury in three warmup games, after being hit on the jaw by a ball last Wednesday against the West Indies. Like the knee, exams showed nothing was broken and he played on.
Opener Lahiru Thirimanne led Sri Lanka with 56 but his teammates failed to build partnerships.
All four Australians who bowled spin — Glenn Maxwell, Adam Zampa, Nathan Lyon, Steve Smith — took wickets.
Australia has won all three of its warmups, while Sri Lanka has lost its two.
England will head back to the Oval on Thursday for the tournament opener against South Africa full of confidence after thrashing Afghanistan by nine wickets.
Afghanistan was all out for 160 in 38.4 overs, and England reached 161-1 in the 18th over.
Jason Roy punished poor Afghanistan bowling by plundering 89 not out off 46 balls, including 11 boundaries.
He and Jonny Bairstow combined for 77 before Bairstow was out for 39 off 22 balls.
Roy combined for 84 with Joe Root, who was 29 not out. Roy finished off the Afghans with his fourth six.
Bowler Mohammad Nabi top-scored for Afghanistan with 44 from 42 balls, including three sixes. But a poor total was already in store at 92-8, and his scoring lifted Afghanistan to 160 before Nabi was dismissed by Jofra Archer.
Archer claimed 3-32 in 34 balls. Part-time spinner Joe Root also got three.
In other good news for England, its fastest bowler, Mark Wood, was cleared to play South Africa on Thursday. Wood pulled up with a left ankle injury on Saturday.
In the last warmups on Tuesday, Bangladesh and India meet in Cardiff, and the West Indies and New Zealand play in Bristol.