Stephens, preparing for her U.S. Open title defense, beat fifth-seeded Elina Svitolina 6-3, 6-3.
Mertens broke immediately and superbly used her backhand to save a break back chance and extend her lead to 3-0 opening up the court for her second consecutive break.
Svitolina won the Rogers Cup previous year when it was held in Toronto. Of course, she's a strong player.
The third title in the season brought Simone Halep over 500 thousand dollars in prize money and 900 ranking points, which will allow the Romanian Monday to increase their lead from her nearest competitor up to 1800 points.
The top-seeded Halep hammered 19 winners and won 77 percent of points on her first serve in the 88-minute contest.
This will be flawless practice for Halep as the final Grand Slam, the US Open approaches.
As for Stephens, the new World No.3 can't be too disappointed in defeat.
Number 15 seed Barty ended an impressive run from Kiki Bertens, beating the Dutch number one 6-3 6-1.
AP Analysis: On enthusiasm, Democrats have advantage
Election handicappers see Democrats as having a good chance of taking back the House in November's midterms. The 2016 loss was particularly troubling for House Democrats, and anti-Pelosi fervor within the party grew.
After a shaky start, the French Open champion won three straight games to take the first set and dominated the second set behind pinpoint serving and stout defence.
"I couldn't believe that it's over, " Halep said.
"I've gotten through some tough moments, had some mental lapses, but I recovered well. I'm happy to be back in North America and playing on hard courts, and I'm hoping for some more good results", Stephens said. It's three finals on the year for me.
Halep, the 2016 tournament champion, was playing her third match in the past two days after earning two wins on Thursday over Anastasia Pavlyuchenkova and 38-year-old Venus Williams.
Stephens to play with the number one player of the world Simona Halep. For sure, I've improved significantly in the last years on changing the surfaces. "I pushed her. on her backhand; then I could just receive a shorter ball, an easier ball for me to open the court", the world No. 1 was quoted as saying on the WTA's Web site.
Then she went home to Australia for a little break before getting back to work. After I took it, I felt more confident. "I have a few goals that I still want to reach". "Actually when I started the tournament, I was really down with my return because I couldn't feel it". "I played very well for a set and a half".
Halep, quoted on the WTA website, added: "I just want to give my best tomorrow and to enjoy the final because it's a great thing after a long break to play final at a big tournament".