Mortlock has turned down "significant overseas approaches" to sign a three-year deal with the Australian Rugby Union with a view to playing in a third World Cup, in New Zealand next year.
The 32-year-old centre is in his 13th season with ACT Brumbies and is the leading points scorer in the history of Super Rugby.
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Hide AdRebels head coach Rod Macqueen, who gave Mortlock his Test debut against Argentina ten years ago, said: "Stirling is yet another addition to our growing team of players with strong leadership skills.
"The opportunity to play out his career in Melbourne, and not overseas, is great for both Australian rugby and Stirling as he helps us build the foundations of the new club."