The Magpies have only taken one point from their first five matches of the 2018-19 League Two campaign, with that disappointing start leaving them bottom of the table.
Nolan has been relieved of his duties in the wake of Saturday's 3-1 defeat by Lincoln City, while assistant manager Richard Thomas has also left the club.
"This is a decision we've made with the best interests of Notts County at heart. While the beginning of our 2018-19 campaign has been bitterly disappointing, the board and I believe results and performances throughout 2018 have not been good enough overall,' Notts County owner and chairman Alan Hardy said in a statement.
"The last eight months have seen us win only eight of our 28 Sky Bet League Two matches, a run which saw us surrender our position in the automatic promotion places before suffering defeat in the playoff semi-finals.
"Our away form has also been a continuing concern, with only two league victories recorded on the road since the turn of the year.
"Ultimately, a continuation of this form would not allow us to meet our targets for the season and, having conceded 16 goals in our last five matches and being without a win in our last 10 competitive games, we feel now is the time to move forward with a new manager at the helm.
"Everybody knows of my desire to restore the fortunes of the world's oldest Football League club and I have invested substantial monies to strengthen our playing squad in the summer.
"Our supporters have responded magnificently by turning out in their droves but, unfortunately, their efforts have not been backed up by performances on the pitch.
"This club will nevertheless always owe Kevin a huge debt of gratitude for his work in saving us from relegation from the EFL when I first arrived in January 2017. It was a truly remarkable turnaround and we wish him and his team every success in the future."