Mitchell has scored nine tries in 34 Super 12/14 games for the Reds while he also picked up six tries in 10 appearances for Australia in the 2005 season.
Force head coach John Mitchell was delighted to have captured the former Australia Under-21 captain, who will link up with the likes of Cameron Shepherd, Matt Henjak and recent big-money signing Matt Giteau in Perth.
"He's an outside threat, who is very quick and has a good left-foot kicking game," Mitchell said.
"He'll be part of a fantastic combination in the back three - whichever role he plays."
The player himself added: "I'm looking forward to catching up with Nathan (Sharpe), Junior (Pelesasa), Matt (Henjak), Shep (Cameron Shepherd) and Gits (Matt Giteau) at a team which has shown plenty of promise this year and will definitely have a good crack next year."