A goal in the third minute of stoppage time from Leatherhead striker Kev Terry saw off Whitstable Town 3-2 and cemented a spot in the Ryman Division One South play-offs.
The north Kent hosts took an early lead before Tanners scored twice to go in front early in the second half, only for the never-say-die Oystermen to set up a tense finish with a fine late equaliser.
Leatherhead manager Richard Brady gave a start to his transfer deadline day signing Richard Avery, with the former Margate player quickly establishing a central midfield holding position for his new club.
Tanners almost made the perfect start when, with less than 90 seconds on the clock, Avery fed a ball through to an advancing Ian Draycott, whose fierce 30-yard drive came back off the post.
But it was 18th placed Whitstable who snatched the lead after 11 minutes - the Tanners defence failed to close down Charlie Smith, who latched onto a loose ball to fire home a well struck shot from the edge of the box.
Leatherhead’s Tom Parkinson posed an early threat down the right, with Tom Bradbrook heading a sweeping cross to goalkeeper Luke Watkins.
But on 18 minutes, Parkinson got round his marker to deliver an inch-perfect cross for Ian Draycott to head home at the near post.
Draycott had an opportunity to put Leatherhead in front after 38 minutes. Bradbrook fed him with a delightful ball down the middle but the former Maidstone forward got his foot under it and the shot went high over the bar.
Bradbrook missed a good chance to give Tanners the lead on the stroke of half-time after he beat the Oystermen’s offside trap to sprint clear of his marker from the half-way line.
However, Watkins came off his line to narrow the angle and Bradbrook fired wide from the edge of the area.
Tanners pressed forward and took the lead from a corner after 50 minutes - Darren Smith picked out Avery who headed home at the near post for a debut goal.
Yet it was hard to believe this was Whitstable’s third match in six days. The hosts never showed any signs of flagging against the second-placed visitors.
Town equalised after 75 minutes, with Macaulay Murray’s long range free kick beating an unsighted Deren Ibrahim to set up a tense finish.
Whitstable almost retook the lead a minute later, with a cross whipped in from the right sent crashing against the post.
But Tanners had the last word, when deep in stoppage time, sub Mohamed Eisa worked his way past two defenders before picking out an advancing Terry to stab the ball home from close range.
Tanners make a speedy return to north Kent on Saturday when they visit Herne Bay.
Leatherhead: Deren Ibrahim, Ryan Cooper, Dan Johnson (Ryan Mahal 34), Richard Avery, Jamie Coyle, Jerry Nnamani, Tom Parkinson (Mohamed Eisa 78), Darren Smith, Tom Bradbrook (Theo Fairweather-Johnson 49), Ian Draycott, Kevin Terry.
Unused subs: David Canning, Paul Semakula.