The 14-year-old saved her best for last, delivering a gutsy and precise performance on her third and final run and was rewarded with a huge score of 93.18 on a sun-soaked day at La Concorde.
Japan's Cocona Hiraki won silver and Britain's Sky Brown took bronze. Hiraki also won silver and Brown bronze at the competition at the Tokyo Games three years ago.
A new champion was guaranteed after Tokyo gold medalist Sakura Yosozumi failed to qualify for the final during preliminary rounds, which were held earlier in the day.
"I'm so excited right now," Trew told reporters. "It’s just crazy.
"It's definitely nerve-wracking with that audience out there."
Being only 11 years old when skateboarding was first contested at the Olympics in Tokyo, she did not compete there but has been on a meteoritic rise since, including winning two X Games championships in the park discipline.
A new champion was guaranteed after Tokyo gold medalist Sakura Yosozumi of Japan failed to qualify for the final during the preliminary rounds, which were held earlier in the day.