Second-half goals from Bobby Traynor and Ryan Woods saw Kingstonian secure a 2-0 win and condemn Leatherhead to relegation after just one season in the Ryman Premier division.
Tanners will end the season in 19th, after drop zone rivals Harrow Borough and Hastings United secured surprise wins against play-off contenders AFC Hornchurch and Lowestoft Town.
Leatherhead manager Richard Brady named an unchanged line up for this must win match, while K’s boss Alan Dowson gave a rare start to goalkeeper Jake Whincup and handed a debut to midfielder Matthew Warren.
There was little sense of urgency in the opening 45 minutes as Tanners failed to conjure up any sort of threat, while the 10th-placed visitors sat back and waited for Leatherhead to come at them.
A lacklustre opening half was marked by one moment of controversy after 20 minutes. Leatherhead’s Harry Ottaway won possession on the edge of the Ks penalty area and was barged off the ball by a covering defender. The crowd behind the goal screamed for a penalty but the referee calmly waved play on.
Whatever Brady said to his players at half time made an immediate impact, as Tanners came out for the second half looking far more determined to produce something that might save their season for them.
Tommy Hutchings burst clear within a minute of the restart and found himself through in a one on one against Whincup.
Hutchings picked his spot and fired low, but Whincup made a fine save with his legs before Greg Andrews fired over the follow up from a tight angle.
Tanners' best chance of breaking the deadlock came after 52 minutes, when any one of three of their players could have scored.
A poor defensive clearance fell to Hutchings, who turned his marker before cracking a blistering shot against the Ks bar. Ottaway picked up the rebound and forced Whincup to make another great block before Leatherhead captain Mark Simmons blasted his follow up shot well wide of the goal. With Tanners pushing further forward, Kingstonian looked more of a threat on the counter-attack, especially after the half-time introduction of their leading goalscorer, Bobby Traynor.
The Ks goal-ace combined well with Louis-Rae Beadle to beat a Leatherhead offside trap after 56 minutes, but Beadle fired wide from distance when Traynor was better placed to score.
That miss was quickly forgotten however as the same two players combined for Kingstonian’s opening goal after 73 minutes.
Ks were awarded a free kick just inside the Leatherhead half after a foul on Matt Pattison. The ball was delivered into the Leatherhead penalty area, headed down by Beadle and driven home by Traynor.
Pattison should have doubled Ks lead a minute later, but fired into the side netting with only the keeper to beat.
With Tanners visibly tiring, Kingstonian pushed forward in search of another goal. Warren burst through the defence to force a good save from Chico Ramos but there was nothing the Leatherhead keeper could do to keep out Ks second after 87 minutes.
Tanners pushed men forward in a desperate search for an equaliser, but when play broke down there were four Ks players ready to break at the half way line against one lone defender. With Ks pouring forward, Woods had the simple task of tapping in past Ramos.
The result sees Leatherhead make a quick return to Ryman Division One South after a season full of missed opportunities.
Leatherhead Chairman, Peter Ashdown, has quickly stepped in to give his full backing to his young manager. Richard Brady can now spend the summer reshaping his squad for next season. Leatherhead: Ramos, Coyle, Gross, Dean, Nnamani, Simmons, Thompson (Dolby 60), Holness (Francis 60), Ottaway, Hutchings, Andrews (Terry 75).
Unused sub: Manners, Richmond.