After a superb first period, the Tykes had looked to be cruising to a comfortable victory, but the visitors had other ideas, taking their hosts all the way to the wire with some inspired rugby.
The Yorkshire side took control from the very kick-off, opening the scoring as early as the fifth minute when hooker Rob Rawlinson crashed over for a try, converted by Duncan Hodge.
Saracens were hit with another blow some three minutes later, fullback Dan Scarbrough showing tremendous pace to find his way to the line and make it 12-0.
The visitors eventually got off the mark in the 14th minute, fly-half Andy Goode kicking a penalty following a Leeds indiscretion.
Goode helped put his side further into contention in the 23rd minute, scoring a try in the corner after some concerted pressure from the Men in Black.
But that action spurred the Tykes into action and they finished the half in devastating form, seizing the initiative with tries from flying wing Phil Christophers and back rower Dan Hyde.
Neither score was converted but Tim Walsh did kick a penalty in the 40th minute to stretch the Tykes' lead.
Goode cut the Tykes lead with an early penalty after the break and that spurred a remarkable second half comeback from the Londoners who looked down-and-out at 28-8 down.
They stormed right back into contention with tries from Johnston and Castaignède – both converted – and a penalty from Goode.
With the scores tied at 31-all, Leeds made an important breakthrough, a fine flowing move allowing Scarbrough to score his second try of the match.
Then, in the 78th minute, Leeds pounced again to seal the victory, Hodge charging down a kick from Goode and touching down the ball over the tryline.
Despite a furious final assault from Saracens, Leeds were able to hold on for victory.
For Leeds Tykes:Tries: Christophers, Hodge, Hyde, Rawlinson, Scarbrough 2Cons: HodgePens: Hodge, Walsh 2
For Saracens:Tries: Castaignède, Goode, JohnstonCon: Goode 2Pens: Goode 4