Seeded fourth in the West, Seattle (16-18) played its regulars most of the game in advance of its playoff series with defending champion Minnesota that begins Friday. The Storm have reached the postseason a WNBA-record nine consecutive seasons.
With their seventh straight loss, the last-place Mercury (7-27) recorded its worst season in franchise history, surpassing an 8-26 mark in 2003. They missed the playoffs for the first time since 2008.
DeWanna Bonner had 13 points to lead Phoenix.
Camille Little had 14 points and eight rebounds, and Shekinna Sticklen added 10 points for the Storm, who finished the regular season with three straight victories.
Candice Dupree, in just her third game after knee surgery, had 10 points and three rebounds in 21 minutes.
Krystal Thomas, who was cut by both Seattle and Phoenix by signed by the Mercury just five days into the regular season, had a career-best 18 rebounds, her franchise-record fifth straight game of 10 or more rebounds.
The Mercury, with just Olympian Diana Taurasi out, took advantage of early Storm turnovers, taking a 16-7 advantage. However, Seattle ran off 16 straight points in the second quarter and led 32-28 at the break.
The Storm also started the second half on an 11-4 run to open up a 43-32 lead with 6:09 left. Seattle led 53-44 going into the final period.
Taurasi missed 24 games this season, much of it with a hip flexor.