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.