The 24-year-old, who has made 275 appearances for the Stones, will remain at the Gallagher Stadium until the summer of 2018 at least.
A product of the club`s own youth system, Mills made his first-team debut in August 2010 and in the past five years has made the left-back position his own.
The current record appearance holder since the club re-formed, he looks set to add many more to that tally over the next two and half seasons.