Player of the match Anika Learoyd cracked 80 off 110 balls and Claire Moore chipped in with 33 as NSW were bowled out for 215 in 48 overs at Allan Border Field on Sunday,
In reply, star Fire opener Georgia Redmayne posted 97 off 122 balls to give Queensland a genuine shot at victory.
But disaster struck in the 41st over with Queensland 7-173 and still needing 43 more runs for victory.
Redmayne was at the non-striker's end when Nicola Hancock smashed Lauren Cheatle's delivery straight back at the bowler.
Cheatle dropped the return catch, but the ball deflected off her hands and onto the stumps, catching a stunned Redmayne well out of her crease.
The Fire's tail didn't last much longer, with NSW players celebrating wildly when Queensland were bowled out for 194 in the 47th over.
It marks the first time NSW have lifted the Ruth Preddy Cup since 2018-19.
"You don't want dropped catches, but that result ended up being ideal for us," Cheatle said of the run-out.
"I can't say that I meant it, but it's probably the best hot spot I'll ever have.
"I'm over the moon. It's kind of hard to put into words what the group feels right now.
"I'm just really proud of this group."
Cheatle (2-32), Sammy-Jo Johnson (2-36), Sam Bates (2-41) and Georgia Adams (2-33) all played important roles with the ball for NSW.
Queensland were left kicking themselves after slumping from 2-135 in the 30th over to be all out for 194.
Redmayne was in hot form for the Fire, but Sianna Ginger (36) and Charli Knott (14) were the only other batters to make it to double figures for Queensland.
NSW veteran Sarah Coyte now has 10 WNCL titles across three different states.
The 33-year-old won six competitions with NSW between 2010 and 2015, one with South Australia after that, then a further two with Tasmania.
Her latest triumph is extra special, given the long wait between drinks for a NSW organisation accustomed to success.
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