The No.2-seeded Radwanska, who was treated during the match for a knee injury, was clearly far from her best, making 18 unforced errors in the first set alone. She never really found her range while the No.20-seeded Pennetta had hers down pat, cruising to a 62 61 victory in an hour and 13 minutes.
It was Pennetta's 10th and - at the Premier Mandatory level - biggest WTA title. Of her previous nine, one was a regular Premier event (Los Angeles in 2009) and eight were at the International level.
"I'm happy. Really happy," Pennetta beamed afterwards. "I think I need a few days to realize this. Right now I'm too calm, I think. I called my dad, and he couldn't breathe. I told him, 'Papa, breathe!'
"After so many years and so much work and everything, this is the moment I was waiting for. And it comes when you least expect it. In the beginning of the tournament I never expected to be the champion, or even be in the final or semifinal. I was just here trying to play my best tennis.
"For me, this is something I was waiting for a long time."
A year ago, Pennetta was just coming back from a six-month wrist surgery lay-off - after falling out of the Top 100, she reannounced herself with a run to the semifinals of the US Open, and has built on that in a big way already this year, reaching the Australian Open quarterfinals and now winning this.
The Italian will rise from No.21 to No.12 now. Her career-high is No.10, which she set back in 2009.
Pennetta's win over the No.3-ranked Radwanska was also the seventh win of her career over a Top 3 player. The first six of those Top 3 wins came at Zürich in 2008 (No.1 Jelena Jankovic), Cincinnati in 2009 (No.3 Venus Williams), Sydney in 2011 (No.2 Vera Zvonareva), Beijing in 2011 (No.1 Caroline Wozniacki), Dubai earlier this year (No.3 Radwanska) and right here just a few days ago (No.2 Li Na).
Radwanska talked about her injury in her post-match presser. "It actually happened a few days ago, but I didn't expect it was going to be much worse today," she said. "I really tried everything I could to push myself and play 100% today, because this is the final, but unfortunately I was in too much pain.
"It's just the worst thing for a player to not be able to give 100% in the final of a big event."
Despite that, Radwanska had a fantastic fortnight. She reached her first final of the year, in fact her first in almost six months, and she also took out a pair of very in-form players in the last two rounds.
"It was still a good two weeks for me. My first final here, a big event, and still a good result.
"Of course I will have a lot of treatment and do everything to be ready for Miami."