The Magpies sit 10th in League One, but lost to rock-bottom Hartlepool on Saturday and have won just two of their last 11 games in all competitions.
Members of Curle's backroom staff, Colin Lee and Colin West, have also lost their jobs.
Chairman Ray Trew said in a club statement: "This obviously is not a decision that we have taken lightly and certainly not one that I have taken any pleasure in reaching, I have a tremendous amount of respect for Keith Curle both as a man and a manager."
Curle was appointed last February and led the Magpies on a club-record 22-game unbeaten run away from home and appeared to be building a promotion campaign this season.
But poor form since the start of December has seen County slip out of the play-off picture, although they remain only five points off the top six with games in hand.
However, Trew suggested that he has acted after fans voted with their feet at recent home games.
Trew said: "Since Keith first came in, just under a year ago, we, as a board, have backed him to bring in the players that he felt could move Notts County onto the next level and felt that he assembled a squad capable of challenging for promotion this season.
"However, recent displays have unfortunately not lived up to expectations and our poor disciplinary record this season has only further hampered our progress.
"Attendances at Meadow Lane have dropped in recent weeks and this has made this a business decision as much as a footballing one, because fans are obviously key to the club and their support is essential."