Cardiff signed Johnson from Wycombe Wanderers for £275,000 in 2006
Influential centre-back Roger Johnson has moved from Cardiff City to Birmingham City for £5m.
The 26-year-old has put pen to paper on a three-year deal which will keep him at St Andrews until the summer of 2012.
Johnson is the third centre-back to join City's ranks this summer following Scott Dann and Giovanny Espinoza.
"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," said Johnson.
"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
"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."
The arrival of Johnson breaks the Blues' transfer record for a defender.
The newly promoted Premier League side had two bids for Johnson rejected before Cardiff gave up the battle to keep their key defender on Wednesday.
Their previous bid failed despite meeting Cardiff's valuation because the full total was dependant on too many variable factors, while the Bluebirds had also wanted the majority of cash up front.
Johnson, who was named in the Professional Footballers Association's Championship team of the year, was arguably the hottest defensive prospect outside the top flight.
His performances for the Bluebirds last season were the bedrock that Cardiff's Championship play-off charge was built on.
But the 2008 FA Cup finalist, one of Cardiff's heroes who starred at Wembley in their 1-0 defeat by Portsmouth, was in tears when Dave Jones' men missed out on a top-six place with 15 minutes of the season remaining.
Johnson also attracted interest from north-east rivals Sunderland and Middlesbrough, as well as Premier League strugglers Portsmouth.
He was a bargain-basement signing by Bluebirds manager Dave Jones in the summer of 2006, when Cardiff beat south Wales rivals Swansea City to capture him for £275,000 from Wycombe Wanderers.
"It will be good value for a player we invested £275,000 in," Cardiff chairman Peter Ridsdale told BBC Sport Wales on Wednesday.
"There is a sell-on clause of profitability to Wycombe Wanderers and they'll be getting what they're owed out of this deal.
"But obviously that still leaves substantial funds for us and we will wheel and deal in the transfer market as best we can.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.