Williams has a wealth of experience under his belt, having helped the Lions to League One promotion in 2017 and has been capped three times for the Republic of Ireland.
The ex-MK Dons had options to remain in the second tier following his release from Millwall, having played 29 times as Gary Rowett's side finished 11th in the 2020-21 campaign.
However, the chance to help the Blues finally remove their League One shackles at the fifth time of asking proved more attractive for the 34-year-old.
Williams told The News: ‘I had opportunities to stay in the Championship.
‘But I think, for the project that is at hand here, it was too hard to turn down.
‘If you can have success then why not do it at a team that's looking to push on?
‘I think we'd been successful at Millwall but had then stayed in the same league.
‘So here, you can make a name for yourself at a club and make a mark for when you do leave.
‘The club is definitely on an upward curve.’
Williams bolsters Pompey’s central-midfield options, with not one previously under contract before his arrival.
Tom Naylor, Andy Cannon, Ben Close and Bryn Morris all left at the end of last season.
Williams is confident he can provide the nous and know-how required in Cowley’s engine room.
And being a left-footer, he believes that attribute will provide the Blues with balance in the middle of the park.
He added: ‘I won't be box-to-box anymore but I usually play in a one or a two.
‘Whatever the manager decides, I'm happy to play. I've always been a defensive midfielder that likes to foray forward the odd time.
‘There aren't as many left-footed players out there so hopefully it can bring some balance to the team.’