Currently assistant to David Ripley as well as batting coach at Wantage Road, Sadler will step up to lead the first team in the 2022 season.
Ripley takes charge of his final first team game this week when Northants travel to Essex to play their final LV= Insurance County Championship encounter of the 2021 campaign.
Ripley has been in charge since the summer of 2012, when he took over the reins from David Capel.
With Ripley stepping aside, Sadler will immediately take up the head coach's role, with Chris Liddle being promoted to his assistant as well as continuing in his role as bowling coach.
The pair are set be supported by Graeme White, who is currently working as a coach with the County second team, as well remaining a white ball first team player with the Steelbacks.
Sadler has been assistant coach at Northants since January, 2020.
He has previously enjoyed interim stints as head coach at both Derbyshire and Leicestershire, and the 39-year-old also took charge of the Steelbacks in this summer's Royal London One Day Cup.
As a player, Dewsbury-born Sadler began his career at his home county Yorkshire, before going on to play for Leicestershire and Derbyshire, amassing more than 5,000 runs in all competitions.
He has concentrated on his coaching career for the past nine years, and last year completed a Masters of Sports Directorship.
Northants chief executive Ray Payne said there was little debate that Sadler was the right man for the job.
“When we brought John on board at the end of 2019 there was always a feeling that he was someone we wanted to keep around long term,” said Payne.
“He’s been an excellent addition to the club and I’m looking forward to seeing what he can do as our head coach.”
The club is now likely to appoint a new batting coach at some point in the future.