"I'm feeling fitter than ever and I want to put the injury nightmares behind me," Johnsen told Villa's official website.
"I feel fine. I had a problem called `Jumper's Knee' which I had problems with. But that's fine now. I've come through the medical and I'm just looking forward to getting started.
"Villa were mentioned quite early in the summer and when the name came up again, I was quite happy to discuss the move.
"There was interest from other clubs, but this was my first choice.
"I'm 33 now, but I can play on at the top level. It's up to you how long you want to work and stay fit and I want to play."