The Magpies have lost their last nine League Two outings to leave themselves 22nd in the table, just one point clear of the relegation zone.
County last picked up a point in the league in October, and their dreadful run of form has led to the club relieving Sheridan of his duties.
"Notts County FC can confirm that John Sheridan is to leave his position as manager with immediate effect," read a brief statement on the club's official website.
"The club will be making no further statement at this time."
Notts County's next match sees them take on Morecambe on Saturday.