The 40-year-old, who played in two World Cups for Australia, had won three of the 14 games since replacing Kevin Nolan at the end of August, while Barnsley's 4-0 FA Cup-defeat by Barnsley turned out to be his final final on Saturday . game.
With County in the 22nd place in the third standings, club chairman Alan Hardy found that the situation was untenable.
"Harry's passion and dedication as a coach is unquestionable," Hardy said in a statement.
"Unfortunately, things simply did not work for him here at Meadow Lane.
"The results were not good enough and the performance is a constant concern, and we see no reason to continue with something that we think can not work."
The club added assistant coach Warren Feeney and the first team analyst Paul Murphy have also left with immediate effect.
Kewell joined the League Crawley Town league, where he was the first Australian to select a professional English side in May 2017. (Reporting by Hardik Vyas in Bengaluru Editing by Christian Radnedge)