Austin, who has played 220 times for Harriers and made it into the National League North team of the season, steps up a division to the National League Premier with County.
Harriers manager Russ Penn said: “Sam has been a fantastic servant to the club, and we’ll only ever have good things to say about him. Notts County triggered his release clause and Sam himself said he felt it was the right time for him to move on. Given those factors, we can’t stand in his way.”
Geraldo Bajrami, who was out of contract at the end of last season, has also signed on at Meadow Lane after the two clubs agreed compensation. Meanwhile, Omari Sterling and Jaiden White have also departed.
Meanwhile, Stafford Rangers have announced that Ben Forrest has agreed a deal for the coming season.
The former Birmingham City centre-back suffered a bad injury against Port Vale last pre-season and only returned to the substitutes’ bench towards the end of the season.