The Blues had been trailing to Wycombe with 32 minutes remaining – yet turned the Fratton Park clash on its head to triumph 2-1.
The winner dramatically arrived in the eighth minute of time added-on, with Conor Shaughnessy heading home Jack Sparkes’ right-wing corner.
For Mousinho, it represented another notable League One scalp over the last four weeks, joining Peterborough, Barnsley and Wigan.
And he feels his players are finally coming to terms with their talents – and Pompey’s potential.
The Blues’ head coach told The News: ‘I think Wycombe are an excellent side, they will compete in the top six this year with some really good players.
‘Wigan were pretty good at the weekend as well, as are some of the sides we have played recently, yours Barnsleys, Derbys and Peterboroughs.
‘Although the league doesn’t have the Sheffield Wednesdays and Ipswichs in it, it’s got a lot of clubs underneath that can really, really compete with each other and that’s what we are seeing this season.
‘What this latest result says, most of all, is we are starting to believe in ourselves a bit more.
‘If I rewind to the beginning of last month and the Peterborough game, I thought we were superb.
‘In the first half against Derby we were okay, we were in the game, and at half-time I asked the players at half-time to believe in themselves as much as we do as the staff - we just think they are superb.
‘One of the big stand-outs in the first half at Derby was Alex Robertson, chalk and cheese to his second-half performance. Then if you look at what he did against Wycombe, that’s what we are trying to get into the players.
‘You know you have that ability, go and play, show us what you show us in training every day. The players have started to listen to us and it is coming together.
‘The most important thing is we take a huge amount of that belief, but also then go “Right, we’ve got a very, very tough test again on Saturday. We have to dust ourselves off, go again and get back at it on the training pitch”.’
With Oxford United beating 10-man Shrewsbury 3-0, a point would have seen Pompey lose top spot.
As it was, victory at the death maintained their one point advantage over the U’s after 11 League One games.
He added: ‘That was an added bonus. We can’t do anything about other people’s results, but the one thing we can control is our performance.
‘You can’t always guarantee you are going to win games, but you can control your own performance, you can control the effort and work-rate – and we did that.’