Mitchell burst onto the Super Rugby scene in 2004 and won his first international cap the following year against South Africa.
The 29-year-old will link-up with the Cote d'Azur club on a two-year deal following the Australia Test season.
And he insists the time was right to move abroad and experience new cultures at Toulon, where he will link up with close friend Matt Giteau.
"I felt the time wasn't right in that period of my life, whereas now I just feel it's the right time for me," said Mitchell.
"Its a great opportunity to experience a different culture, to challenge myself in a different environment.
"This (moving to Toulon) has been in discussion for a little while now."
Mitchell was a high profile omission from this week's Wallabies training camp but he insists the snub was not a part of his decision.
And he has left the door open to a return Down Under to resume his Test career following his stint in France.
"Robbie Deans made it very clear to me the reasons as to why I was left out of the squad and by no means does that mean there's no way for me to work my way back into the squad in the next six weeks or so.
"I certainly don't want this to be seen as me turning my back on the Wallabies or the Waratahs because playing for Australia means everything to me and I've really enjoyed my time at the Waratahs.
"But as far as looking any further about World Cups, that hasn't even been a thought in my mind."
Mitchell's Waratahs face Warren Gatland's men on June 15, before the first Test a week later.