Mitchell, 18 stone and 6ft 5ins, will link up with Connacht following the conclusion of Taranaki's current campaign, and may offer a selection headache to Ireland coach Eddie O'Sullivan.
Irish-qualified by virtue of his grandparents, Mitchell could force his way into international contention by impressing in Connacht's Celtic League campaign, which begins on Saturday at home to Cardiff, and the knockout stages of the European Challenge Cup, for which he will be eligible.
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 him, he has an Irish place to chase down."