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.

The original Megaupload file-sharing site proved enormously popular and was thought to have about 50 million users a day at its peak.
It was shut down in 2012, following accusations by the US that copyright pirates made heavy use of it.
The new service would reinstate the accounts of all former users of the original Megaupload, said Mr Dotcom.
Plans for the revived Megaupload were revealed in a series of tweets by the German-born entrepreneur, who now lives in New Zealand.
He said it would be formally launched on 20 January - five years to the day since his home was raided by police investigating the "massive" amount of pirated material available via Megaupload.
Since the raid, Mr Dotcom has waged a lengthy legal battle to fight accusations that he profited from others abusing Megaupload and to recover the assets seized at the time.
He is also fighting attempts to extradite him to the US to face trial over his control of Megaupload.
The tweets suggest that the new Megaupload service will involve the Bitcoin virtual currency.
However, it is not clear whether it is being used as a payment system or as a way to track what people share via the service.
It is also not clear how accounts created on the original service will be reinstated.
Mr Dotcom wrote that emails would be sent to former users when the service was ready for them to rejoin it.
Mr Dotcom has also said the 2017 version will:
As well as sending messages about the new service, Mr Dotcom issued an appeal to former Megaupload staff to get in touch to help bring it to life.
In the years since the raid, Mr Dotcom has dabbled in New Zealand politics and launched a separate file-sharing and storage service called Mega.
He resigned from Mega's board of directors in 2013, and since then the company's other investors have taken control and run it separately.
UKIP have won six seats on Thurrock Council, missing out on being the largest party by a single vote.
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Net entrepreneur Kim Dotcom plans to relaunch the Megaupload web storage system in January 2017.