The 21-year-old gold medal favourite got off to her usual explosive start and led for almost the entire race with Serwa tucked in behind her.
A third one-two in a freestyle skiing event at the Sochi Games ensured Canada would top the medals table in the sport, ahead of the United States, with four of the 10 golds.
Anna Holmlund of Sweden was assured of the bronze when the fourth finalist, Ophelie David of France, lost control about a third of the way down the course and crashed out.
It was a second Olympic disappointment for multiple World Cup champion David as the 27-year-old crashed out in the quarter-finals on the event's debut in Vancouver.
Serwa also failed to justify her favourites tag in Vancouver four years ago and set out as if on a mission on Friday by setting the top time in the seedings run ahead of David and Thompson.
The seeding runs were interrupted for 10 minutes when fog and rain restricted visibility but the skies had cleared in time for the start of the knockout heats.
As usual in a sport that is known for bumps and spills there were plenty of crashes.
German Anna Woerner was taken off the course on a stretcher after a violent fall during the quarter-finals, her left leg appearing to give way on impact as she hit the icy landing slope.
Chile's Stephanie Joffroy also needed a stretcher to get off the slope after falling in the next quarter-final but a wave as she went past the crowd indicated she was not too badly hurt.
Safety concerns were high after Russian free skier Maria Komissarova fractured a vertebra and dislocated her spine during practice on the same course at the Extreme Park last week.
There were few surprises in the heats apart from Swiss world champion Fanny Smith failing to progress after losing control while leading her semi-final.
Thompson stormed through the knockout rounds winning her three races by significant margins and she quickly raced clear of the field in the medal decider.
Her gliding skills and long but stable jumps make her a formidable opponent even in the rough and tumble of the pack and skiing out front she never looked like being caught.