West Lancashire League
A victory over the third-bottom side would have seen Town go five points clear at the top with four games left.
Manager Lee Ashcroft started with two up front, Jack Preston joining Brad Carsley, with Jordan Bowen playing just in behind.
Town started OK with Carsley heading Mark Woods’ cross wide after just 12 minutes, but four minutes later, a Southport corner to the edge of the penalty area was volleyed home to put them 1-0 up.
After 38 minutes a free kick was lofted into the Longridge area and, after failing to clear their lines, Town went two down as the ball was stabbed home from six yards.
Former Southport player Mark Jeffers was introduced, in place of Preston at half-time as Ashcroft looked to get the ball wide and provide the ammunition for Carsley.
Jeffers looked bright and Town started the half seeking the goal to get them back in the game.
The closest they came was a glancing header from Danny Morris that rolled wide.
Joe Melling was replaced by Jordan Tucker on the hour, as Ryan McKenna moved up front with Carsley, but it was all in vain.
The Southport keeper remained untroubled, aside from showing good hands when saving a Carsley free-kick on 76 minutes.
It was all over two minutes later when Southport outnumbered the Town defence and fired a third goal into the corner of the net.
The final 12 minutes were played out in the grim belief that Longridge had missed an opportunity, but Southport had played well and fully deserved their three points.