Fowler and her team from Brandon, Man., scored a deuce in the first end and added two more points in the third. Simpson conceded the game before the eighth end was completed at the Iceberg Skating Palace.
"We're absolutely, completely thrilled to come out on top of this event," Fowler said. "We're all absolutely elated. We thought we'd played well all week and we were confident we could do it but we knew we'd have to play well."
It was the 10th time that Canada has won gold at this event while it was the first-ever medal for Italy. The Americans took the bronze with a 6-4 win over Sweden.
American skip Lyle Sieg defeated Canada's Alan O'Leary 9-4 to win the men's senior title. The U.S. led 6-1 after five ends and added three more points in the seventh before running the Halifax-based team out of stones for the victory.
"Silver feels not great but we had a good week," O'Leary said. "It was the thrill of a lifetime having the Maple Leaf on our back, but (the Americans) have been hot all week and full credit to them for becoming world champions.
"We lost to the better team today, so congratulations to them."
New Zealand beat Denmark 6-4 for the bronze medal.
In mixed doubles, Calgary's Kalynn Park and Charley Thomas dropped a 9-4 decision to Norway's Kristin Skaslien and Magnus Nedregotten in the bronze-medal game.
Hungary edged Sweden 6-5 for the gold.