Frawley, who confirmed he was leaving the Demons last Wednesday, was free to join the club of his choice as an unrestricted free agent with the Hawks lodging paperwork on day two of the free agency window.
It is a major coup for the Hawks, who lost star forward Lance Franklin as a restricted free agent at the end of last season.
Melbourne is likely to learn its compensation for Frawley's departure on Monday, given that the Western Bulldogs' compensation for losing midfielder Shaun Higgins to North Melbourne was announced late on Friday, the day his departure was confirmed.
It is expected that the Demons will receive a first-round pick, which would be pick No.3 in this year's NAB AFL Draft.
The 26-year-old will further bolster the reigning premier's defensive stocks and shapes as a long-term replacement for Norm Smith medallist Brian Lake, who signed a new one-year deal on Thursday.
Frawley was linked to Geelong and Fremantle before nominating Hawthorn as his preferred destination.
The Cats will instead deal with Melbourne to sign key forward Mitch Clark.
The 26-year-old will further bolster the reigning premier's defensive stocks and shapes as a long-term replacement for Norm Smith medallist Brian Lake, who signed a new one-year deal on Thursday.
Frawley told the club's website Lake's two flag wins in two seasons since moving from the Western Bulldogs as a free agent played a part in his decision.
"I'd be happy to put my hand up for one of them - that's enough," he said.
Frawley did spare a thought for the Demons, where he clocked up 139 games after being drafted with pick 12 in 2006.
"I'm sad to leave (Melbourne) but you have to do what's best for you," he said.
"I found it really hard, especially when it came down to the crunch of actually making the decision."
Frawley was linked to Geelong and Fremantle before nominating Hawthorn as his preferred destination.
The Cats will instead deal with Melbourne to sign key forward Mitch Clark.
Hawthorn recruiting and list manager Graham Wright said Frawley was a great pick-up for the club.
“Having recently turned 26, James is a quality defender in the prime of his career,” Wright said.
“He is a team-orientated person who has got great character and excellent values.
“James is a fantastic addition to our list, and will complement and enhance our defensive unit extremely well.”
