Gov. Jan Brewer, who had removed Mathis from her post, called the court's decision "deeply regrettable."
"I am disappointed, certainly," the governor said Thursday after the court ruling. "More important, this misguided ruling bodes ill for the integrity of redistricting in this state."
Brewer accused the redistricting commission of following an unconstitutional redistricting process, conducting too much of its business behind closed doors and disregarding mapping criteria.
"With its reinstatement of the IRC chairwoman, the Supreme Court has averted its eyes from the Commission's misdeeds," Brewer said in a statement. "The Chairwoman's actions to meet in secret, arrange critical votes in advance of meetings and twist the words and spirit of the Constitution have been forgiven -- if not endorsed outright."
The court decided the governor failed to demonstrate that Mathis showed "substantial neglect of duty, gross misconduct in office or inability to discharge the duties of office," the Arizona Republic reported.
Brewer's decision to remove Mathis came after the commission redrew congressional districts that Republicans claimed unfairly favored Democrats, CNN said.