The 28-year-old has spent the past decade accumulating more than 300 appearances for Peterborough, Sunderland, Salford and Bristol Rovers.
He was a regular starter at the Memorial Stadium last term and has featured in each of the League Two side's four games this term, although he was substituted at half-time last Saturday after Rovers shipped four goals in the first half of a 4-1 defeat at Exeter.
That proved to be his last appearance for The Pirates before the termination of his contract, with manager Joey Barton deeming him surplus to requirements as he attempts to reduce the size of his squad.
County boss Mackay was delighted to capitalise, saying: "We spoke at length to him about the club, our ambitions and why we wanted him to be part of our plan and we are delighted that he has chosen to join us.
"Jack joins us in the prime of his career, brings great experience to the group, and we look forward to seeing him pull on the Ross County jersey."