The Rebels will now play at Step 2 of non-league football for the first time since being relegated from the old Isthmian League Premier Division in 2001, and will now join the National League for the first time since its inception in 2004.
At a sun-drenched Walks Stadium, the stage was set for a titanic tussle between the Linnets, who finished the season as runners-up to champions Hereford with 100 points, and the Rebels, who finished the season only a point behind with 12 wins from their final 14 league games.
A huge crowd descended upon the stadium with kick-off pushed back 30 minutes due to crowd congestion on police advice – and when the game started, anything between 3,000 and 4,000 people had managed to get inside to watch the action.
The hosts were quicker out of the blocks and created the first chance. Slough were unable to clear on the edge of their own box, and striker Michael Gash hit a fierce drive which cannoned back off the crossbar.
Minutes later, Cameron King tried his luck from distance, but his effort lacked power and Jack Turner saved easily.
Slough were almost presented with an opening thanks to an under-hit back pass from Ryan Jarvis, but Lynn keeper Alex Street rushed out to clear with Perry Coles closing quickly.
Warren Harris headed straight into the arms of Street as Slough grew into the game, and the Slough winger saw a cross well cleared after he had worked his way into the box.
When the opening goal arrived, it came for the hosts against the run of play. Frazer Blake-Tracy’s long free-kick from out wide was headed down by Gash, and Ryan Fryatt helped it across goal into the path of Toby Hilliard, who poked the loose ball home.
And the Linnets very nearly doubled their lead almost immediately as a high cross from Simon Lappin was missed by Turner in the Slough goal. Gash then hooked the ball across the face of goal but Guy Hollis got a crucial header in to deny Jarvis, with Hilliard heading the loose ball wide.
Coles remained a lively presence in attack for Slough, and he beat Lappin to a long ball forward from George Wells but fired wide from an acute angle.
But Slough deservedly levelled just a few minutes before the interval. James Dobson’s corner was only half cleared and was returned to the winger by Lee Togwell. Dobson chipped the ball in and Chris Flood rose above Gash to send a header down and past the reach of the despairing Street.
Slough went close to taking the lead early in the second half as Flood slipped the ball through to Coles, but the striker’s shot hit a post after he showed good footwork to beat Blake-Tracy.
Flood later found himself in front of goal thanks to a pass from Harris, but he scooped the ball over under pressure from a defender.
Slough continued to press, and substitute Williams laid the ball off neatly for Dobson, who took a touch before shooting, which allowed Jarvis to block.
King’s Lynn appealed in vain for a spot kick for handball when substitute Leon Mettam attempted a scissors kick, but it was blocked by Scott Davies.
Then, with just a minute remaining came Slough’s magic moment.
Davies showed great composure in the middle of the field to evade several challenges and play the ball to Dobson on the left wing. Dobson looked up and played an early ball into the box for Williams, who controlled brilliantly before turning past Tom Ward and firing past Street to spark wild scenes of jubilation amongst the hundreds of Rebels supporters who had made the long trip.
Desperate for a response, Lynn piled bodies forward and created one final chance as Fryatt’s cross was headed over his own bar by Davies – but Lappin’s corner was wasted, with the ball arcing out of play before coming back in.
Turner claimed the ball one final time from a Lynn flick-on and the final whistle sounded to confirm Slough’s promotion at the end of a quite remarkable season – a second under the management of Neil Baker and Jon Underwood.
King's Lynn Town Line up
Alex Street, Cameron Norman, Frazer Blake-Tracy, Simon Lappin, Ryan Fryatt, Ryan Jarvis (c), Cameron King, Craig Parker, Michael Gash, Toby Hilliard (Leon Mettam 80), Ryan Hawkins (Tom Ward 62). Unused substitutes: Dion Frary, Eoin McQuaid, Tom Siddons.