The required 10,000 signatures were gathered to force a vote on a recall of council members F. Dick Jones, Don Bankhead and Pat McKinley.
They were criticized for not immediately demanding the release of city-owned surveillance video showing the beating of Thomas.
Thomas was a mentally ill homeless man who died following a violent confrontation with police. Two Fullerton police officers have been charged in connection with his death.
The signatures have been validated, and the City Council will certify them at a Feb. 21 meeting.
The council has discretion on the election date, and they can choose to hold a special election or to consolidate it with the June 5 presidential primary.