The 24-year-old tight-head prop made his Ospreys debut in 2006 and has made 35 appearances for the region.
Mitchell made his Welsh debut against Canada during the 2009 summer tour of North America and has gone on to win eight caps.
With Adam Jones sidelined, Mitchell played in the opening four matches of the 2011 RBS 6 Nations before suffering a dislocated shoulder in the victory over Ireland.
But after finding his path to the No3 jersey blocked by Jones at the Ospreys he has taken the decision to join the Aviva Premiership out-fit in the search of regular rugby.
“I’m really looking forward to it,” said Neath-born Mitchell.
“It’s going to be a fresh start for me and a move that is right for me and my career at this stage. I’ve had six good years with the Ospreys, but I felt the time was right to try something else and signing for Exeter is a great opportunity for me.
“I’ve always said Adam’s the world’s best tight-head, so when he signed for another three years I knew it was time to move on.
“I appreciate certain games are highly pressurised, but I’m competitive and will not improve being on the bench.
“It’s going to be my first crack at English rugby and the Premiership. I’ve spoken to a few boys about it and they’ve said it’s tough, but that it is also a very good standard. “Obviously I’ve come up against English sides before with the Ospreys and with Wales, so I know a little bit about the players and the teams.”
Exeter head coach Rob Baxter was delighted to secure the capture of Mitchell and reckons he can futher enhance his international credentials at Sandy Park.
“It’s great to have him on board,” said Baxter.
“It’s unfortunate for him that he picked up the injury pretty late in the Six Nations but everyone who watched him will think he’s done very well, he’s certainly proved a few points and certainly progressed his international credentials.
“Hopefully we can help him continue with his international ambitions, as we’d like to with some of our other players.
“We’ve got a scrum that’s improving all the time, our props are playing really well, but you can never have too many quality guys there.
“He’s also a very good age as well, so he’ll keep improving and developing and I think we’re looking forward to seeing how he goes over the next couple of years.”
