Mansour scored the first of his two tries in just the third minute when he was able to get outside Tom Trbojevic and did well to ground the ball for the opener.
In a tough, tense battle that ebbed and flowed through much of the first half, Manly were next on the scoreboard in the 24th minute when Justin Horo pounced on a Kieran Foran kick after a period of sustained pressure on the Panthers' line.
A Peta Hiku penalty goal on the stroke of halftime put the visitors ahead 8-6 at the break.
But with just a two-man bench for the second half, Manly eventually ran out of puff with Matt Moylan spearheading the Panthers to a confidence-boosting win.
Steve Matai, who scored a late consolation try, was Manly's best in his return from a shoulder injury, with the visitors struck hard by the loss of Brett Stewart pre-game with a groin complaint, while Peter Wallace (knee) was a late withdrawal for Penrith.
Manly were unable to respond to chairman Scott Penn's midweek warning that they needed to produce results or Toovey's tenure would come under consideration.
- AAP