World number one Williams, 32, now has 62 career WTA titles, but lifted the trophy in Cincy for the first time in her sixth attempt.
She won nine of the last 10 games against Ivanovic to roll to victory in just over an hour.
Williams has won both of her summer hardcourt events played in the United States, taking the Stanford title last month. In between she reached the semi-finals in Toronto, losing to her sister Venus.
"I was just so excited for this final," Williams said. "I just really felt like I had nothing to lose. So I was able to relax.
"It truly feels good. I didn't expect to come here and win or have such a good summer. I've just been trying to go with the flow and be positive and it's working out."
The American finished with a love game against Ivanovic, a former world number one who will now return to the top 10 in the rankings for the first time in five years.
Williams took advantage of her scheduling good luck after playing the afternoon semi-final on Saturday, while Ivanovic had to work for nearly three hours the night before to battle past Maria Sharapova.
"It's definitely very, very quick, you know." Ivanovic said of the turnaround. "Despite all that, I really tried my hardest. She just played too good today.
"This is the biggest final I've been in in a while and it felt good to be part of it," she added. "I just want to keep working hard and keep improving in the areas that I can.
"There are still a few points in my game that I feel I need to work on for the US Open and also for the end of season. Definitely exciting times."
Williams now stands 38-6 on the season and has not lost in any of her five finals.
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However, she hasn't made it to the championship match in a Grand Slam -- a trend she now feels could be reversed at the US Open.
"I won Stanford but I definitely didn't play as well. Then in Montreal, I got to the semis but didn't play as well as I did here," she said.
"On my first day of practice here something just clicked," added Williams, who had fallen to Ivanovic at the Australian Open in January.
Williams fired a dozen aces and broke the Serbian four times. The American was broken early to trail 3-1, her only real danger in the brief match.
The top seed levelled with a love game for 3-3 and won the opening set from back-to-back Ivanovic double-faults.
Williams found another gear in the second set, taking a 1-0 lead and breaking her tiring opponent twice more along the way.
Ivanovic and Williams were both playing their WTA-leading fifth finals of 2014. Williams now owns five titles while Ivanovic has three.
"I really felt she was really solid today," said Ivanovic. "Not only on her serve, but on the groundstrokes I felt like she was dominating.
"I did have chances, especially when we started the rallies, but I just couldn't get the serve back."
"It was still a great match even if I lost," Ivanovic added. "I'm looking forward to the US Open, it's the last Grand Slam of the season and the most exciting one. I'm looking forward to trying to do well."