The 24-year-old Mitchell, who lined out for Taranaki against the Lions this summer, is the second foreign-based front rower to link with the westerners in the close season. South African Chris Venter is already in place at the Sportsground.
Mitchell is of considerable bulk at 6ft 5in and 18st, and will link up with Connacht following the conclusion of Taranaki's current NPC campaign.
Qualifying for Ireland by the grandparent rule, Mitchell will not be eligible for the European Challenge Cup pool stages, but should Connacht progress to the knockout rounds, he should expect some game-time - especially after a gradual introduction in the Celtic League.
Connacht manager Tim Allnutt said of the new signing: "We are taking a long-term view on this signing. Hamish is a quality prop who will be a big addition to our squad. He is coming here to further his career and he knows that if everything works out for hime, he has an Irish place to chase down."