James Buchan said he wanted to be able to "look my family in the eye and say, 'that's not who I am'" after Nigel Farage's outfit announced plans to axe indefinite leave to remain.
The Dartford Borough councillor, who won his seat for Reform in a July by-election, said the policy creates a "huge amount of fear and anxiety" and accused the party of having "a pretty unfortunate way of treating people".
A Reform source said: "He got elected in July 2025 as a Reform councillor. He should resign his seat but he won't because he knows he will lose under the Tory banner.
"We look forward to winning this seat back as soon as possible."
In a statement, Mr Buchan said: "I stood for election with the sole aim of working for my community and getting things done for local families.
"Having had the opportunity to see Reform from the inside, I've concluded that the party doesn't really have the experience or ambition to do that."
He added that "relying on rhetoric and slogans isn't going to help real families in communities" like his and "the more I saw of Reform UK, the more uncomfortable I felt to be part of it".
"Things like a proposal to remove indefinite leave to remain from all non-EU residents who are working here perfectly legally is an example that could be devastating for decent people who have built a life here and contribute to our country," he said.
Reform announced, earlier this year, that it would scrap indefinite leave to remain and replace it with new visas that would require migrants to reapply to live in the UK as part of plans to tighten immigration.
Conservative council leader Jeremy Kite confirmed the move on Friday and welcomed Mr Buchan, who represents the Stone House ward, to the party.
"James knows that getting elected isn't about power, it's about service and being supportive of people," he said.