The Blues’ top scorer with nine goals this season got the visitors back on level terms deep into first-half stoppage-time.
Referee Martin Coy added a minimum of eight minutes on to the end of the opening period after two stoppages of play as home fans registered their anger with owner Dai Yongge by throwing tennis balls onto the pitch.
Those disruptions did Pompey no favours as they soon found themselves 2-0 following goals from ex-Fratton Park transfer target Lewis Wing (23 minutes) and Charlie Savage (27).
On-loan Chelsea midfielder Tino Anjorin halved the deficit with his maiden league goal for the Blues just after the half-hour mark. Then up popped the unmarked Bishop to calmly slot home Paddy’ Lane’s perfect pass across goal with the final touch of the first half.
The home dugout were incensed by the timing of the goal as they believed the half-time whistle should have already been blown.
However, Mousinho said he and opposite number Ruben Selles knew exactly how long was left to play as he also hailed Pompey’s patience as they tried to grab an important equaliser before the break.
The Pompey head coach told BBC Solent: ‘We knew that it was going to be eight minutes put up and it was going to be eight minutes 30 (seconds played). We were told by the fourth officials so both benches knew.
‘I don’t think they were particularly happy with that but both benches knew and it was scored before then. You probably see I’m a bit weird like that, I put my timer on!
‘I’m happy with that patient build-up. I want us to stick to the game plan and I know if we get the ball wide, we cross it, we deliver it into the box, we’re a good side.
‘If we force balls into the middle of the pitch, they break on us and Sam Smith nearly scored and punished us (when we did that).
‘We could have ended that point at 3-1 down or even maybe down to 10 men (after Will Norris rushed out of his goal), so we’re got to be a lot better, stick to the plan – and I think if we do then we’ll be absolutely fine.’
Bishop’s timely strike allowed Pompey to go into the break at 2-2. It then handed the visitors the momentum in the second half, with Terry Devlin firing home a third and the Blues’ winner on 58 minutes.
The win extends Pompey’s unbeaten start to the season and sees them open a six-point lead over second-placed Oxford after they could only draw with Wycombe at the Kassam Stadium.