The Aussie striker turned the game in the last quarter of an hour after Mansfield's determined defence, and some great saves by Jason White, looked like earning a draw.
Hartlepool were on top in the first half, though it was Mansfield who threatened first when Richie Barker got in a shot at the end of a good move on two minutes, but home keeper Dimi Konstantopoulos saved well.
Then it was the turn of Town keeper Jason White to be busy. He did well to hold a James Brown header on 17 minutes; and even better with a double save two minutes later at the feet on the marauding Matty Robson.
White was almost beaten on 41 minutes, but after cutting inside from the right to create himself the chance, Brown's blistering shot bounced to safety off the near post.
It was a fast first half, with a lot of hard running on a hot evening, and Hartlepool should really have been in front by the break.
Mansfield came more into the game after the break, but White still produced his best save of the night on 72 minutes when his one-handed stop kept out a close-range volley from Brown that looked a certain goal.
But Porter went on in 62 minutes, and on 77 he grabbed the breakthrough. He kept running after passing the ball forward to Brown on the edge of the area and picked up possession, broke through and slotted his 12-yard shot into the bottom corner.
Mansfield battled hard to come back and created a string of half-chances for the first time in the game. They won a handful o corners, too, as they pressed Hartlepool back.
But they couldn't find a way through and, in injury time, Porter settled it with a spectacular striker from the angle of the penalty area way out on the right, his curling shot floating into the top far corner.