Spence made his announcement at a City Council meeting Thursday night.
MORE: West Covina Mayor Mike Spence involved in drug investigation
He said he would officially step down on June 1, but he was voted out and removed from his position immediately.
His resignation comes after he was found unconscious in a hotel room in Costa Mesa earlier this month in what's being investigated by police as a possible a drug overdose.
A new mayor will be announced in June.