"I am really pleased to be here. Birmingham are a massive club and I am looking forward to putting the shirt on to be honest," he told the media.
"Premier League football is the dream at the start of any footballer's career so I am pleased I have been given that chance with Birmingham and I can't wait to get started.
"The aim is to stay in the Premier League with Birmingham and hopefully I will have a big part to play in that.
"I want to do well for the club. At both clubs I have been at I have got on well with the fans and hopefully that will continue here."
It's been reported that the defender will treble his wages at Birmingham. Cardiff are set to receive £2m upfront, £2m in a years time and another million dependent on appearances and Premier League survival. Wycombe Wanderers could earn upto £750,000 in sell on clauses from the deal that took Johnson to Cardiff from Adams Park.
With Darren Purse leaving the club as well this summer Dave Jones has lost two out of the three centre halves that were at the club last season.