A stunning final day of the season saw the Blues hammer Cheltenham Town 6-1 and clinch the League Two crown.
Previously only Wolves, Preston, Burnley and Sheffield United had claimed the title in each of the English game’s top four divisions.
Now the name of Pompey sits proudly alongside them, following a remarkable day at Fratton.
With Plymouth and Doncaster failing to win, Cook’s men took control to seize top spot on goal difference.
And Pompey’s boss revealed he never had doubts about the majestic outcome.
He said: ‘I got to the ground early and believed we would go and win it.
‘If you are a manager and don’t believe, how can you make anyone else believe?
‘I have believed it all week. That result at Mansfield was huge, that Clarke sending off was huge, and people don’t remember that.
‘You don’t remember little moments in season where you get beaten at Morecambe, you don’t remember it, but you actually remember we are champions – and that memory will stay with us all.
‘People say we’ve had low points, I call it part of the season, that is where you are trying to go.
‘There are two parts to the race – the start and the finish.
‘In the middle you have bad moments and we’ve had them, make no bones about it, but Saturday will without doubt live long in the memory of Portsmouth Football Club.
‘It doesn’t get any better for Pompey.
‘People spoke about Notts County but Saturday left it absolutely for dead.
‘You win 6-1 at home and might not win the league.
‘But we did and are now champions.’
Daniel O’Shaughnessy had set Pompey on their way when he turned Enda Stevens’ cross past his own keeper.
Yet it wasn’t until the second half when the Blues discovered their rhythm and raised the performance levels.
Man of the match Kyle Bennett sparked a spell of four goals in 13 minutes, with Kal Naismith, Jamal Lowe and Gareth Evans also netting.
Despite James Dayton pulling one back for the Robins, Naismith bagged his second to etch those responsible into the Pompey record books.
Cook added: ‘I went absolutely mental at half-time.
‘We have an arrogance about us at times when the game is a little bit simple, you have to put teams away, it was summed up by Kal Naismith’s free-kick in the first half.
‘I don’t do that, I don’t entertain stuff like that and Kal knew it. When he came up to me after scoring his first goal he gave me a calm down tablet.
‘As you can imagine, the whole of half-time was spent between me and Kal debating how the game should be played!
‘But we are champions – and we played like it.’