Matt Tubbs bagged the resulting penalty in the second minute before Jed Wallace made it 2-0 from range on 55 minutes.
Teenager Jack Thomas’ first senior goal kept the game alive on 67 minutes, but it wasn’t enough to stop Portsmouth ending a run of six away games without a win and earning their first ever league win against Mansfield.
The game was only 50 seconds old when Chris Clements gave the ball away in midfield and Pompey broke quickly.
Matt Tubbs fed Jed Wallace to his left but, as he tried to finish, he was fouled by Sutton seven yards from goal.
The penalty was awarded and Sutton red-carded, Tubbs then sending Lenny Pidgeley the wrong way from the spot with two minutes on the watch.
The visitors used the extra man well in the first half and almost made it 2-0 on 24 minutes when Pidgeley had to beat away a shot from Craig Westcarr after a loose pass from Callum Elder.
Stags’ best chances fell to Rakish Bingham, but he fired two efforts off target and saw Paul Jones get a hand to a close-range finish from a Matt Rhead flick.
After Jones had to make a save to deny Junior Brown just after the break, Wallace doubled Portsmouth’s lead on 55 minutes.
He let fly from 30 yards and the ball took a wicked bounce as the keeper dived and sailed high above him into the net.
Pidgeley superbly turned over a Dan Butler blast from a half-cleared corner.
But Stags were back in it on 67 minutes as teenager Thomas celebrated picking up four Player of the Season awards pre-match by curling a beauty into the top right corner from just outside the box on the left.
Pidgeley beat away a Danny Hollands shot near the end as Stags threw everything forward in vain.