This is just the latest staff member Quan has lost. Deputy Mayor Sharon Cornu resigned two months ago, saying she felt she was ineffective in her role.
Quan replaced her with Jerry Brown's former top aide, Anne Campbell. The mayor's office says Cohen will not be replaced.
The mayor will lose one more staff member because of budget cuts. She needs to cut 20% of her office's budget to help the city deal with an estimated $26 million deficit.