Cardoso beat compatriot Filipe Toledo in an all-Brazilian final as the South Americans dominated the contest.
They provided three of the four semi-finalists with only Gabe Kling of the United States breaking the party.
Australian hopes were ended heart-breakingly in the quarter-finals when Cardoso edged out Chris Friend and Hizunome Bettero shaded Mitch Crews - both by less than half a point out of 20.
Cardoso sealed the final with late points for a series of gouges, and said he was delighted with victory.
“I was here and came 5th when Adriano (De Souza) won," Cardoso said. "This is so amazing for me. I was really, really nervous when Filipe (Toledo) scored the interference. I only needed a small score and I kept falling. Then I got that 7-point-ride and I knew that I had won and I was so happy.”
Willian Cardoso (BRA) 10.16 def. Filipe Toledo (BRA) 9.50
Filipe Toledo (BRA) 15.00 def. Gabe Kling (USA) 12.07
Willian Cardoso (BRA) 14.40 def. Hizunome Bettero (BRA) 11.27
Filipe Toledo (BRA) 16.34 def. Nat Young (USA)
Gabe Kling (USA) 15.67 def. Evan Geiselman (USA) 12.37
Willian Cardoso (BRA) 14.90 def. Chris Friend (AUS) 14.40
Hizunome Bettero (BRA) 16.16 def. Mitch Crews (AUS) 15.76