The 23-year-old England hopeful completed a quartet of scores in just 21 minutes as Gloucester ended a run of three successive home league defeats.
But the west country club had to withstand an impressive Leicester fightback that saw Tigers move from 30 points adrift after 33 minutes to just six behind midway through the final quarter.
Uncapped Thorley’s feat matched four-try efforts by former Bath wing Tom Voyce in 2001 and ex-Wasps speedster Christian Wade four years ago.
Gloucester had a bonus point in the bag after 26 minutes, although a much-changed Leicester rallied strongly from 36-6 adrift just before the break.
Billy Twelvetrees added four conversions and a penalty for Gloucester, while Zack Henry kicked two penalties and substitute George Ford landed three conversions and a penalty as he inspired Tigers’ ultimately fruitless quest to avoid an 11th league defeat this season.
The game began with a flurry of scoring as Gloucester wiped out a second-minute deficit following Henry’s penalty by breaching Tigers’ defence from their first attack.
Fly-half Danny Cipriani was heavily involved in the build-up play before Harris made an incisive break and sent an unmarked Simpson sprinting over.
It was a blistering start by the home side, but Leicester dug in and started to show some impressive resilience, and a spell of pressure ended with Henry kicking a second penalty, cutting the gap to 12-6.
Cipriani pulled the tactical strings impressively for Gloucester, cleverly mixing his kicking and running game, but Thorley then upstaged him during a devastating eight-minute spell.
His second try showcased his power from close range, then he completed his hat-trick after collecting full-back Jason Woodward’s kick into space, before adding a fourth touchdown when he finished impressively from 40 metres.
Leicester did not know what had hit them, and they conceded a sixth try before the interval when Gloucester number eight Jake Polledri stole possession on halfway and sent Harris on a clear run to score.
Even though Potter weaved some magic through a fine try that Ford converted, Leicester remained at sixes and sevens, trailing 36-13 following a first-half when Thorley ran them ragged.
Ollie Thorley has scores four tries inside 30 minutes ?
Another brilliant finish!
— Rugby on BT Sport (@btsportrugby) August 30, 2020
Ford was joined for the second period by his fellow England half-back Ben Youngs, and they kick-started Leicester into action.
Tigers had success during the third quarter, keeping Gloucester scoreless, and they managed a try themselves when back-row forward Taufua went over and Ford converted.
Gloucester had already made a number of changes by this point, including Cipriani being replaced, and they needed to regather themselves as Tigers found an impressive patch of form.
But Henderson’s try, converted by Ford, narrowed the gap to just nine points, with Leicester having established strong momentum, yet Twelvetrees’ late penalty and Evans’ try eased Gloucester nerves.
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