Top seed Sabalenka triumphed 7-5 6-2 to finish the year with a ninth straight win and back-to-back trophies following her success two weeks ago in Ostrava, where she was also victorious in the doubles alongside Mertens.
The Belarusian took an hour and 42 minutes to overcome the second seed, moving her level with Simona Halep for the most WTA Tour titles in 2020.
Mertens had a chance to serve out the opening set after breaking Sabalenka for the second time at 4-4, but she failed to do so and dropped her next service game as the match turned on its head.
Sabalenka made it three straight breaks early in the second and she somehow hung into a rally before producing a powerful backhand to secure another piece of silverware.
"Hopefully there's many more titles to come for our team … it's a tough period and I really appreciated having the opportunity to play here," said Sabalenka.
"Everyone, thank you so much. It's been an amazing week and I'm really happy to finish the season with a title."
Ranked a career-best number 21 in the world, the 18-year-old sensation is atop the singles draw at the Upper Austria Ladies Linz this week.
Raducanu's remarkable run at Flushing Meadows has generated an avalanche of publicity and requests, but she insisted tennis will remain her top priority.
"I made it very, very clear to every single person in my team that I was not going to cancel one training session or practice session for any off-court commitments," Raducanu told reporters.
"That was non-negotiable for me. I wanted to make sure that that is my priority and it is, so everyone is clear about that. But it's just managing my time with the commitments around that."
Raducanu has a bye in the first round and will face the winner of the opening-round match between qualifiers Kateryna Kozlova and Wang Xinyu.
The teen split with coach Andrew Richardson after the US Open and remains without an official adviser but said she plans to have someone in place by January's Australian Open.
That was not something she would have anticipated worrying about during last year's tournament in Melbourne, when she was still a full-time student.
"Only March the 18th was my first session on court in 2021," she said. "And so January, February, March I was literally just sat on my desk staring at a wall for nine hours a day.
"So I feel like where I am now I just need to really take it all in and enjoy because looking back at how far I've come it's pretty surreal.
"I just really need to enjoy it because when I was in the beginning of year I would have never thought this was possible. I'm kind of just really living in the moment right now I feel."
