For the top-seeded American, the triumph marked her first in six visits to Cincinnati and sent a message that she is in top form heading into the year's final grand slam, where she is the two-time defending champion.
"It's just amazing to finally win here," said Williams, who fired a dozen aces and broke her opponent four times. "The fans were amazing and it's so wonderful to be here."
After falling behind early, Williams broke her ninth seeded opponent's serve in the sixth game of the first set to pull even and then broke again four games later to wrap up the opener in 38 minutes after consecutive double faults from Ivanovic.
A year removed from losing the Cincinnati final in a third set tiebreak to Victoria Azarenka, the world number one left nothing to chance in a dominant second set.
Williams broke Ivanovic to open a 3-1 lead in the second set and then relied on her powerful serve to carry her the rest of the way.
It marked the fifth title of the year for the 32-year-old American following wins in Stanford, Rome, Miami and Brisbane.
The Cincinnati tournament is the last of the key tune-ups ahead of the Aug. 25-Sept. 8 U.S. Open in New York.
For 26-year-old Ivanovic, a former world number one who was chasing her fourth title of the year, her run to the Cincinnati final will put her back in the top 10 of the world rankings for the first time in five years.
"It's been a great week for me in Cincinnati," said Ivanovic, who was coming off a nearly three-hour win over fifth seed Maria Sharapova in Saturday's semi-final.
"I really want to congratulate Serena. I think I got a lesson on how to serve today. Maybe after you retire you can give me some tips."
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