The 40-year-old, who played in two World Cups for Australia, had won three of 14 games since he replaced Kevin Nolan at the end of August, with County's 4-0 FA Cup defeat by Barnsley on Saturday proving to be his final game.
With County floundering in 22nd spot in the third-tier standings, club chairman Alan Hardy felt the situation was untenable.
"Harry's passion and commitment as a coach is unquestionable," Hardy said in a statement.
"Unfortunately, however, things quite simply were not working out for him here at Meadow Lane.
"Results have not been good enough and performance levels are a continuing concern. We see no reason to continue with something we don't think can work."
The club added assistant coach Warren Feeney and first-team analyst Paul Murphy have also left with immediate effect.
Kewell had joined County from League Two rivals Crawley Town, where he had become the first Australian to coach a professional English side in May 2017. (Reporting by Hardik Vyas in Bengaluru Editing by Christian Radnedge)
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