King, 35, earned his only All Blacks cap against Wales in November 2002.
He has also spent time with the Chiefs in Super Rugby and had spells at Stade Francais and Clermont in France.
Hardy, 20, who played for Wales in last year's Under-20 World Cup, previously faced Jersey in the British & Irish Cup while with Carmarthen Quins.
"Regan is a fantastically gifted player who will be the perfect addition to our squad, helping to bring on the likes of Brendan Cope and Sam Katz, and offering a different perspective to our backs, particularly in terms of their attacking skills," Jersey boss Harvey Biljon said.
"People will talk about Regan's age, but that's not an issue for me - he has made well over 20 appearances for the Scarlets this season and has expressed real enthusiasm about playing for Jersey once we get next season under way."
King was part of the Scarlets side that came to the island in pre-season last summer.
"You could see there was a lot of potential and I believe the results this season back that up - being only two wins from making the play-offs was a significant achievement," said King.
"I'm seeking to add something to the squad to help the club push on next season and am really looking forward to getting started."
Championship side Jersey have signed former New Zealand centre Regan King and scrum-half Kieran Hardy from Pro 12 side Scarlets.