The Canadians had seemed to be headed to their first ever Women Sevens Series title but Australia scored three late tries to deny their dream at Twickenham Stadium, the home of English rugby.
Canada came into the tournament second in the overall standings, with the Australians third. They left tied in second behind New Zealand at 76 points.
The loss dropped Canada's record in Series finals to 0-3. The Canadians lost both other finals to New Zealand, 36-0 in Atlanta in February 2104 and 33-24 in Amsterdam during the inaugural 2012-13 campaign.
The women played their first five matches of the tournament at the Twickenham Stoop, home of famed club Harlequins, before moving next door to Twickenham Stadium for the third-place playoff and final, at the end of Day 1 of the men's sevens tournament schedule.
That meant playing before a record crowd of 77,033, many of who were in fancy dress. Organizers had called for a galactic theme, following the success of last year's monster motif for fans.
Sharni Williams put Australia ahead 5-0 early after Ghislaine Landry had made a try-saving tackle on the other side of the field.
Kayla Moleschi replied for Canada after Australia lost the ball at the breakdown and Landry, completing a fine team move, touched down just before halftime for the 48th try of her career. Landry converted her own try for a 12-5 lead at the half.
Landry almost scored again from the second-half kickoff but she was bundled out at the corner flag. Moleschi increased the lead to 17-5 after a big tackle by captain Jen Kish dislodged the ball near the Australian line.
Then the Aussies mounted their comeback, despite having a player sent to the sin-bin for a high tackle.
Charlotte Caslick crossed at the corner to narrow the deficit to 17-10 with 2 1/2 minutes remaining. Nicole Beck, taking a fine offload from Amy Turner, then scored but Australia still trailed 17-15 after the conversion was missed.
Green, a 22-year-old speed merchant, put the game away with the clock at zero.
Both teams came into the final unbeaten, with Canada winning all five matches and Australia winning four and drawing one.
The Canadian women were coming off a disappointing sixth-place finish last month on home soil in Langford, B.C. They finished third at the three previous tournaments.
Series leader New Zealand scored a try with no time remaining to edge the U.S. 24-19 in the third-place playoff Saturday. Strike runner Portia Woodman scored her 50th try of the season for the Kiwis.
New Zealand came into the tournament, having won the four previous stops on the circuit and carrying a 36-match winning streak. But the Kiwis lost both days in London, 19-15 to unheralded Spain in pool play Saturday and 24-5 to Australia in Sunday's semifinal.
The season wraps up next weekend in Amsterdam.
The top four in the overall standings qualify automatically for the 2016 Olympics with New Zealand having already booked its ticket despite the weekend losses.
Canada dominated the first half of its semifinal against the U.S. but only led 12-7 after a late solo-effort try by Victoria Folayan cut the deficit. The American speedster scored again early in the second half, slicing through the Canadian defence for a converted try and a 14-12 lead.
Harvey, picking up the ball at the breakdown, gave Canada a 17-14 lead that proved to be the final score. Canadian Elissa Alarie was hurt on the play and had to be helped off.
Britt Benn and Ashley Steacy scored first-half tries for Canada. Landry added a conversion.
Canada opened the day by dispatching Russia 26-5 in the quarter-finals.
Bianca Farella, Alarie, Harvey and Landry scored tries for Canada, which racked up 19 unanswered points in the second half. Landry added three conversion.