Mayor Rahm Emanuel says it was a "mutual decision" between he and his communications director that she look for a new opportunity.
He denied he fired Chris Mather, calling her a "valuable part" of the team who did an "excellent job."
"Come the holidays, come the end of the year, people make choices," said Emanuel. "She made the choice and we discussed it and it's a mutual decision."
Mather surprised many when she sent a departure note from her personal Gmail account on Thursday afternoon, saying she was leaving "knowing we accomplished what we set out to do."