Cook County Board President Toni Preckwinkle demanded her chief of staff John Keller’s resignation after he allegedly engaged in inappropriate behavior.

“As President of the Cook County Board of Commissioners and as a woman, it is important to me to foster a workplace that is respectful, where all people are treated with dignity,” Preckwinkle said in a statement Wednesday. “I have always had zero tolerance for inappropriate and disrespectful behavior.”

Keller’s abrupt departure comes as Preckwinkle will announce Thursday whether she’ll enter the crowded race for Chicago mayor.

Keller became Preckwinkle’s chief of staff in 2017 after working his way up through Cook County government. He supervised all offices under Preckwinkle’s watch.

In her statement, Preckwinkle said she learned on Friday about an allegation that Keller engaged in inappropriate behavior on his personal time. The allegation was corroborated. Preckwinkle confronted him about it, and Keller didn’t deny it.

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He resigned on Tuesday. Keller couldn’t be reached for comment.

In his resignation letter to Preckwinkle, Keller said: “I am honored to have served as your Chief of Staff since January of 2017. It is with mixed emotions that I submit this letter of resignation effective immediately. Thank you for the opportunity to serve.”

A spokeswoman for Preckwinkle would not provide more details.

Thursday, Preckwinkle emailed a memo to the county’s more than 20,000 employees. She reminded them that she’s mandated annual ethics training for all workers, and this year, every county employee had sexual harassment training.

“I will continue to demand that each and every employee understand the prohibitions regarding harassment inside and outside Cook County facilities,” Preckwinkle said in the memo. “I will continue to inform our employees of their duty to report any witnessed or known problematic behavior.”

She listed several government agencies where workers can report harassment.

Preckwinkle is scheduled to announce this afternoon at a Hyde Park hotel her ambitions for Chicago mayor. Mayor Rahm Emanuel announced earlier this month that he is not seeking a third term.

Preckwinkle would join more than 12 candidates so far jockeying for the city’s top political job. They include Bill Daley, the brother and son of two previous longtime Chicago mayors, and Gery Chico, who lost to Emanuel in 2011 and was chief of staff to Richard M. Daley (Bill Daley’s brother).

Kristen Schorsch covers Cook County politics for WBEZ. Follow her @kschorsch.