The former Osprey switched to Sandy Park last summer and has become an integral member of the Chiefs’ pack.
Mitchell, who has won 11 caps in the front row for Wales, suffered an elbow injury against London Welsh last weekend and is a major doubt for the autumn internationals.
But he has secured his immediate future, alongside livewire wing Matt Jess, who has enjoyed previous stints at Newport Gwent Dragons and Ebbw Vale.
“It’s great that these guys are seeing where we want to go and what we want to achieve,” said Chiefs head coach Rob Baxter.
“I know they’re ambitious players who are probably coming into their prime for what they’re going to be as players, so for them to commit nice and early is great for the club and I think it’ll be great for the players.
“I’m 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.”
Mitchell was a part of Warren Gatland’s Rugby World Cup squad and featured in the 81-7 victory over Namibia in New Plymouth.
He was again selected for the RBS 6 Nations but did not feature as Wales claimed the Grand Slam.
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