Sweden’s Anna Holmlund took bronze after 37-year-old French veteran Ophélie David crashed out of medal contention, ending her bid to become the oldest ever Winter Olympic gold medallist.
“I'm really glad our freestyle team has been so successful here. It's so great that we can finish it off with a bang,” said Thompson, whose gold after Canada also had one-twos in both men's and women's moguls.
Serwa, the 2011 world champion, added: "It's the water. We just have a really good programme and we're really supportive of each other and push each other.
"We've been watching each other all week and getting fired up every time Canada gets on the podium."
Thompson and Serwa seized control of the race from the start, and their one-two ensured that Canada finish top of the freestyle skiing table with four golds, one more than the USA.
For Thompson, women's World Cup champion in 2011/12, the taste of victory was made all the more sweet by the presence of her friend and team-mate on the podium.
“I'm really, really excited, I'm so glad I get to share this moment with Kelsey up here,” she enthused.
“It's amazing for our country. I'm so proud I could bring it home to Canada and keep it in Canada,” added Thompson, who took the title won by her compatriot Ashleigh McIvor at Vancouver 2010.
Anything can happen
For Serwa it was a remarkable turnaround, as she very nearly missed out on the final, after trailing at the back of her semi-final until the last jump.
Only a slip by reigning world champion Fanny Smith of Switzerland, who had been leading semi-final until losing her balance, allowed the Canadian to progress.
“In ski cross anything can happen. I never give up until I cross the finish line,” said Serwa.
“That was just one of the works that was in play there.I was in fourth out of the start and all the way down to the last turn.
“[Smith] fell in front of me so I avoided her and at that point it was just a race to the line.”
First for Sweden
Meanwhile, Holmlund won Sweden's first ever medal in ski cross and their first in freestyle skiing for 20 years.
“It's really special to get the first medal in ski cross for Sweden. It's a good beginning for us," said the 26-year old.
“Of course you want to win a medal, but the competition is really hard. It was harder and harder for every race. I still haven’t taken in the fact that I have a bronze medal,” she added.