Prior won the Essex County Council division of Rayleigh West with a majority of 796 votes, taking the seat off the Liberal Democrats.
The Reform candidate also won Sweyne Park and the Grange on Rochford. Farage’s party won all 13 seats up for grabs on the district council – and no one party now has overall control there.
A by-election is likely to be held for both seats.
Farage was asked by BBC Essex last month if all of Reform’s candidates had been vetted in the county.
“I know that our candidates will be held to a higher standard than any of the other parties,” said the Clacton MP.
“That’s because we are the challengers. We are the ones taking on the establishment. Yet we have done a good, thorough professional job”.
Joe Mulhall, director of research at Hope Not Hate, accused Reform of “a systematic failure with the party’s vetting”.