Bottom-placed Southport shocked title-chasing Stevenage with this well-deserved victory.
From the outset it was the visitors who dominated the game, and they had the first chance when Liam Blakeman fired a drive at home keeper Alan Julian.
The visitors should have taken the lead after 20 minutes when Blakeman was fouled in the box by Julian. Referee Connolly pointed to the spot, but Carl Baker saw his effort cannon back off the crossbar and away to safety.
Stevenage had few efforts on goal, but Danny Williams did force keeper Steve Dickinson into a diving save just before the break while Jefferson Louis guided a header agonisingly wide from close range.
Southport piled on the pressure in the second half, and despite a double substitution from home boss Graham Westley, Stevenage were too lethargic to respond.
Baker guided a header against the post after 56 minutes, but Southport finally got their reward five minutes later when Neil Robinson drove home from the penalty spot after Ronnie Henry had fouled Baker.
Henry was sent off for the challenge, and Stevenage looked unlikely to get back into the game with 10 men.
Their performance was summed up three minutes from the end when a hand-ball from Neil Fitzhenry gifted them a penalty which Darryn Stamp blasted straight at keeper Steve Dickinson.