In a game that had everything North End, who began the evening in the drop zone, recovered from 4-1 behind after 39 minutes to snatch a barely believable away success.
Parkin had given them an early lead before the hosts responded brilliantly with Davide Somma's double added to goals from Luciano Becchio and Alex Bruce seemingly putting them in control.
Anyone in attendance at Elland Road who thought the game was over was to be proven badly wrong, though, as North End produced a remarkable riposte.
Parkin, who was applauded by the home support when he was substituted late on, was the star, netting either side of goals from Keith Treacy and Callum Davidson, before Iain Hume added a sixth.
Going into the game on the back of winning only their second game of the season, against Coventry on Saturday, Preston enjoyed a dream start as, after tricking his way down the left, Treacy forced Shane Higgs to parry into the path of Parkin, who tapped into the empty net.
Going behind seemed to spur Leeds on, though, and they soon restored parity as George McCartney supplied a cross which Becchio powerfully headed past Andy Lonergan.
Leeds nearly went ahead moments later, only for Lonergan to deny Somma after he was brilliantly played in by a Becchio backheel.
But the vocal home support did not have to wait too long to see their side take the lead as, after the resulting corner was seemingly cleared, Sean St Ledger ludicrously headed across goal to leave Bruce with the simplest of chances.
Not content with a one-goal advantage, the Whites pushed for another and got it in the 20th minute as Somma's deflected effort beat a helpless Longergan.
Simon Grayson's side were simply irresistible for the next 20 minutes, causing Preston's hapless defence all sorts of problems.
Somma was again on hand to enhance his growing reputation in the 39th minute, cleverly cutting inside from the left wing before unleashing an unstoppable effort into the bottom corner.
North End were not ready to give up without a fight, though, as Parkin reduced the arrears just a minute later after ghosting in behind the Leeds backline.
The Lilywhites picked up where they left off at the start of the second period, with Treacy somehow scoring direct from a corner in the 54th minute to add to the tension.
The game continued to compound the expectations of even the most imaginative fan as Preston made it 4-4 moments later when, after Bruce brought down Hume, skipper Davidson slotted the resulting spot-kick home.
And one of the most astonishing comebacks was complete when Davidson's long throw found Parkin, who showed brilliant control before completing his treble with an excellent half volley.
The visitors were not finished there, though, adding a sixth as Billy Jones' diagonal ball found Hume who looped a header over Higgs into the opposite corner.
And, despite Johnson rattling the post, that proved to be the last goal as Preston saw the game out to claim an astonishing victory.