AFC Totton triumphed 3-0 in front of a Southampton derby crowd of more than a thousand at Sholing on Tuesday evening.
Both sides struggled to create chances during a typically tense but competitive first half.
The Boatmen went close with a back-post header steered over by ex-Totton midfielder Charlie Gunson.
Three minutes before half-time, Totton were awarded a penalty for handball from Tom Blair’s cross.
Scott Rendell coolly sent Ross Worner the wrong way to put the Stags ahead, somewhat against the run of play.
Five minutes into the second period, Worner produced a brilliant save to palm Tony Lee’s volley past the post.
But Totton kept on coming and got their just rewards six minutes past the hour when Luke Hallett darted in at the near post to convert a low centre from Marcus Daws.
Sholing boss Paul Doswell made a triple-change but Totton defended resolutely and remained ready to spring on the counter-attack.
Two fresh-legged substitutes combined to wrap up the win with a stunning goal on 89 minutes.
Charlie Austin controlled a long ball and advanced to the right wing before picking out Ethan Taylor on the opposite flank.
The winger’s first touch put space between himself and his marker, and from 25 yards out, he struck a sweet, curling left-footed shot across Worner and into the net via the foot of the right-hand post.
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