The 26-year-old midfielder had his contract at Colchester United cancelled.
“It’s an exciting time for the club and an exciting time for myself,” Jackson told the club’s official website.
Jackson signed a three-year contract with Newcastle.
“Johnnie hasn’t figured in manager Paul Lambert’s plans and so has been allowed to join Notts County,” chief executive Steve Bradshaw told Colchester’s official website.
“He was Player of the Year for us in the Championship and we hope that this move will help him restart his career at a new club.”