Campbell has been in prolific form since moving to Ninian Park, scoring six goals in five games for Lennie Lawrence's side who are now closing in on a play-off spot.
Bluebirds owner Sam Hammam is desperate to get his club into Division One, and takes his spending past the £10 million mark with the £900,000 he has splashed out on the 22-year-old.
Campbell has signed a three-year deal to stay in Cardiff, and hopes to bang in the goals to secure their play-off spot, and then promotion.
"It was a big wrench to leave Middlesbrough because I'd been there since I was a kid," said Campbell.
"I've been at Cardiff only a few weeks but I've already realised how big the club is. In Sam Hammam they have an owner with real enthusiasm and in Lennie Lawrence they have a man I respect greatly."
Although he is loathed to leave his home town, Campbell believes that the move is right for his career, and also says that Cardiff will become a bigger club than Boro.
"I feel Cardiff are going places and I want to be part of their progress. I truly believe City can be a bigger club than Middlesbrough.
"It's going to be a wrench to leave my home and all my friends and family back on Teesside. But I really feel my career needs this move."
Lawrence has also strengthened his squad with the addition of West Ham goalkeeper Stephen Bywater on loan.
Cardiff have been flying since Lawrence took over from Alan Cork last month, and he hopes this double swoop will send out a message to the rest of his promotion rivals.
"The signings of Campbell and Bywater are a definite statement to the other clubs that we mean business," said Lawrence.
"The capture of Campbell represents a lot of money on top of what Sam Hammam has spent already at Cardiff.
"But Andy has settled in well down here - he's already established a good rapport with our fans. He's going to be a real asset - he's a 22-year-old with genuine pace."
