The Cats and Collingwood were the last two clubs in the race to secure the 26-year-old, with Clark spending time in Geelong at the weekend as he made his choice.
The tall forward negotiated his exit from Melbourne in April this year, retiring after he was diagnosed with clinical depression, and he is seeking a fresh start at a new club.
AFL.com.au has confirmed he nominated the Cats as his preferred destination on Tuesday.
Melbourne’s manager of football operations Josh Mahoney said the Demons accepted Clark’s decision.
“We understand Mitch's position, he has indicated that Geelong would be his preferred club and that he would like us to get the best deal available for him,” he told AFL.com.au.
“We will work with Geelong on what that deal looks like in the coming days.”
The move rules out a direct swap for Magpie Heritier Lumumba, who was linked to the Demons in a swap for Clark.
Cats football manager Neil Balme said the club was delighted Clark had chosen Geelong as his favoured home.
"He has said that he wants to come, so we've got a bit of work to do now," Balme told NAB AFL Trade Radio on Tuesday afternoon.
"But we're certainly pleased with that outcome."
Balme admitted there was risk involved with bringing Clark into the club, considering his injury and mental illness history.
But the Cats were confident they would be able to "mitigate" those risks.
"We're very confident (in Clark's foot). We've got our own people who we've spoken to and they're confident it won't be a long-term problem," Balme said.
Balme also confirmed the Cats were hoping to accommodate Melbourne’s unrestricted free agent James Frawley onto their list in addition to signing Clark.
The race for Frawley is believed to be between the Cats and reigning premiers Hawthorn, with Fremantle a distant third.
The All Australian defender is yet to tell the Demons he wants out, but it is widely expected he will not be playing at Melbourne in 2015.
Balme also said Geelong would look at its options in relation to midfielder Taylor Hunt in the upcoming NAB AFL Trade Period.
"He may not have to go," Balme said.
"But he may well find somewhere to go that better suits him, so we'll see what happens in the trade period."
