It looked as though a dominant Pompey display at Port Vale was going to go unrewarded.
But with time running out, Abu Kamara was tripped inside the box by Conor Grant and Colby Bishop slammed home from the spot.
There was a mixture of joy and relief from the large travelling army behind the goal, with the result keeping the side at the top of the table.
And Mousinho felt that it was a fully warranted outcome following a controlled display for most of the afternoon.
The Blues boss said: “As the game progressed, we were able to open them up and create chance after chance.
“We need to be better in a lot of those moments to make sure we actually put the ball in the back of the net.
“But I thought that we played really well across the 90 minutes and that the victory was thoroughly deserved.
"As the game progressed, we were able to open them up and create chance after chance."
“There were shots that were blocked and there are times when you think it might not come, but you could see that the boys meant business and they didn’t let their heads drop.
“I thought that it was a proper tackle from Conor Shaughnessy in the middle of the pitch and then it looked like a stonewall penalty, so it was a great decision.
“Colby had a few minutes to think about the penalty and what happened a couple of weeks ago, so it showed real mettle to pick the ball up and then put it in the back of the net.
“There was then an unsavoury incident with a fan going for the referee and I’m glad that he was able to get out the way. We don’t want to see things like that.
“It was tough to deal with the late barrage and we naturally sat deeper for the final few minutes, but we defended well and I’m proud of the way we saw the game out.”
There were plenty of questions for Mousinho about forthcoming transfer activity, with the window closing in a few days.
He said: “I genuinely don’t know because I switch off from that as soon as we leave the training ground on Friday.
“Rich [Hughes], Andy [Cullen], Tony [Brown] and Phil [Boardman] deal with that, so I’ll catch up with them and we’ll see how we’re going.”