UKIP has taken the Rayleigh South seat from the Tories.
Former Tory Keith Gibbs beat Conservative John Flack to win by 185 votes.
This was the second of six seats in south Essex to be taken by Ukip on the night.
Mr Gibbs, a retired policeman, said: “I’m elated. I’m proud to be one of the first Ukip county councillors in Essex.”
The loss will be a huge blow for Mr Flack, a prominent Essex Tory, who stood in the division after long-serving councillor Mavis Webster's decision to step down.
For a full list of results click here.