Fardy's signing with European powerhouse Leinster comes after the 30-Test back- rower fell out of favour with Wallabies coach Michael Cheika last season.
The 32-year-old's decision to leave the Brumbies comes after experienced players Stephen Moore, Joe Tomane and Matt Toomua all departed the club at the end of last season and while David Pocock is unavailable on a one-year sabbatical.
Fardy has collected more than 80 Super Rugby caps for the Brumbies since his debut in 2012.
"Scott has been a fantastic player for the Brumbies during his spell with the club," Brumbies chief executive Michael Thomson said.
"Whilst we are obviously disappointed that Scott has decided to move on, we thank him for his service for the club and wish him all the best for the future."
While Fardy's loss is a further blow to the Brumbies, coach Stephen Larkham, who is also leaving the club at the end of the season, says Carter's contract extension is a good sign for the future.
"Sam's re-signing sends out a positive message throughout the club," Larkham said.
"He is a well-respected individual within the playing group and leads from the front."