After a councillor passed away, Reform UK won a by-election to replace Labour in Wolverhampton.
After her passing was revealed in August, tributes came in, even though Sue Roberts MBE was elected in Bilston North in 2023.
Reform UK candidate Anita Stanley narrowly defeated the Greens on Thursday, receiving 652 votes, about 200 more than Labour.
The Conservatives polled 257 votes.
Reform UK Chairman Zia Yusuf described the result as a "stunning victory" and that it showed no Labour seat was safe, while adding the Tories were falling "even further behind".
"Reform is rapidly opening local branches and assembling its ground campaigning capabilities," he said.
"By the time of the English county council elections in May, Reform will be a formidable electoral force."