Clark quit the Demons earlier this year citing depression, but he has since gotten on top of the condition and chose to make a fresh start with the Cats.
The 27-year-old's presence up forward will give Geelong another vital avenue towards goal after the Cats' scoring output diminished last season.
Spearhead Tom Hawkins scored a career-best 68 goals last season, but the addition of Clark will help ease the load.
Clark managed just 15 games over three seasons at the Demons after being blighted by a serious foot injury, but he's hopeful his best football is still ahead of him.
Johnson is looking forward to linking up with Clark, but said the team wouldn't place unrealistic expectations on the West Australian.
"I think he'll definitely add to our team. As long as he's playing his role, we don't expect too much," Johnson said.
"I think if he can just help out with our forward line depth, give Tommy Hawkins a bit of a chop out (that will be enough).
"By the sounds of things Mitch and Rhys (Stanley) are training strongly at the moment, so it's exciting times for us."
Johnson is in Perth preparing for Saturday night's International Rules clash with Ireland at Patersons Stadium.
The 31-year-old is one of the most skilful AFL players when it comes to the Sherrin, and his round-ball skills are also being talked up by his teammates.
Johnson did his best to play down his golden touch.
"I am not sure who talked me up but I'm not that good," Johnson said.
"I guess it is just foreign that the ball is a lot different.
"I've been practising since I was five years old kicking an oval ball, and kicking a soccer ball, or an Irish football, is a little bit different."
Australia's squad features only current or former All-Australians, and Johnson said he was amazed by the depth of talent on display.
"On Tuesday I played up forward, so alongside Nick Riewoldt, Chad Wingard and Robbie Gray. You look around and have to pinch yourself really," Johnson said.
"It has been fun."
