And the Chiefs have also announced fresh two-year terms for wing Matt Jess.
Mitchell, a former Ospreys player who was part of Wales' 2011 World Cup squad, is currently in his second season at Sandy Park.
"I think recruitment and retention are year-round things," Exeter head coach Rob Baxter said.
"They are not something that starts at Christmas or two months before the end of the season.
"It's great that these guys are seeing where we want to go and what we want to achieve.
"I know they are ambitious players who are probably coming into their prime for what they are going to be as players, so for them to commit nice and early is great for the club and I think it will be great for the players.
"I am seeing both of these guys coming on in leaps and bounds, and for us as a club to have them for the foreseeable future is great.
"The more guys you can get tied down, feeling relaxed and comfortable and knowing where they are for a little while creates a nice balance and solidity to the squad."
Jess joined Exeter from Cornish Pirates in 2008, and has so far scored 47 tries for them.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.