The potential move surfaced at the end of last month when Pompey made an official approach for Cook, one immediately turned down by the Chesterfield board as they pushed for promotion to the Championship.
It made little sense to me then and does now but I wondered if there had been some fall out between Cook and Chesterfield, but nothing could be further from the truth.
The two parties have, today, spoke highly of each other. Chief Executive Chris Turner said: "I could not have wished for a better working relationship than the one I have enjoyed with Paul over the last two and a half years and there is nobody more disappointed than me to see him leave."
In reply, Cook said: "I'd like to place on record my thanks to Chris, chairman Dave Allen and the rest of the board for giving me the opportunity to manage such a great club." He also went on to thank and praise the staff, players and fans.
But the announcement from Pompey reveals that it is the opportunity to work for a bigger club that has persuaded the former Burnley midfielder to make the move.
"I'll be moving down to Portsmouth and getting straight to work," he said. "I won't be taking a break until I'm comfortable that things are in place and there are a lot of things to sort out, but this is a new beginning for everyone and I'm delighted to be named Pompey manager.
"I had a great two and a half years at Chesterfield and enjoyed my time there immensely, but being able to manage a massive club like this is something I'm very excited about. I want to steer this ship in the right direction, but a lot of hard work is needed because there is no magic wand in football."
Cook had been at Chesterfield five days longer than Sean Dyche has been Burnley manager. This move means Dyche is now English football's 17th longest serving manager of the 92 clubs in the Premier and Football Leagues.