The party, which was the third biggest group after the last local election, took four seats from Labour, one from the Tories and one from an independent.
The final result at the Little Thurrock Blackshots ward was won by the Conservatives by one vote over the UKIP candidate after three recounts.
UKIP and the Conservatives have 17 seats each, and Labour 14.
There is one independent. The parties are now in discussion about whether a coalition can run the authority. Thurrock Tory group leader Rob Gledhill and UKIP leader Graham Snell said they believed they could work together.
Basildon - where UKIP took two seats, one from Labour and one from the Lib Dems - and Southend-on-Sea councils remained with no one party in overall control.
Harlow Council was held by Labour, and Castle Point and Brentwood councils were held by the Conservatives.
Colchester City Council remains with no party in overall control. The counts for Rochford Council and Epping Forest are taking place on Friday.
The tensions at the Backstage Centre in Purfleet were palpable. Just when the final declaration seemed imminent it went to a third recount and became even more nail-biting.
Thurrock was a top target for UKIP on a national basis, and in the end they fell agonisingly short in coming out level with the Conservatives. Nonetheless, they're jubilant at having picked up six seats, which the leader Graham Snell puts down to his colleagues' hard work on local issues such as housing, jobs and the environment.
Thurrock is now in an unprecedented situation with joint council leaders. Graham Snell and Robert Gledhill agree that there is common ground on about 70% of issues. In particular, they're united in their opposition to any new Thames river crossing coming through Thurrock.
UKIP have won six seats on Thurrock Council, missing out on being the largest party by a single vote.