A hard-fought goalless home draw against Fisher Athletic was enough to save St Leonards from relegation from the eastern division.
Neighbours Hastings Town and Sittingbourne beat Wisbech Town and Corby Town respectively.
Manager Glyn White said: "We're all over the moon. We played well and created as many chances as in any game I've seen this season. It was a good battling team performance and the whole squad, not just the 11 against Fisher, have worked so hard. We really deserve to stay up."
Saints went close on several occasions. Dave Fisher and Dominic Barclay were handful and it was only the bravery and reflexes of Fisher goalkeeper Michael Holder that kept them at bay.
Danny Moody had an early chance and Barclay saw his effort scrambled off the line.
Eastbourne Borough wound up their eastern division home programme with a 2-2 draw against Erith and Belvedere.
Steve White burst through a hesitant home defence to put the visitors ahead in the tenth minute.
Andy Ducille levelled on 32 minutes and Dave Adams made it 2-1 in the 57th. Erith equalised through a Glenn Billenness penalty ten minutes from time after Rob Longhurst floored Marvin Neufville.
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