Boam said he intended to continue representing the Valley ward and would still vote with his former Tory colleagues in the council chamber.
He also told the BBC he had stopped paying for membership of the Conservative Party “a while ago”, but had continued to sit with the group on the council.
Boam said: “My decision to resign is based on the current leadership of the North West Leicestershire Conservative Association (NWLCA) ‘under’ Craig Smith.”
Smith is the county councillor for Coalville but told the BBC that he was not an executive member of the NWLCA.
Boam said he disagreed with the way Smith had been selected to represent the Tories in the Long Whatton and Diseworth by-election on 1 October.
“He got himself selected and there were plenty of other people who live nearer to the ward who wanted to do it,” Boam added.
“Having served as an elected member for more than 15 years, I find I can no longer continue under the association’s current management style and do not wish to be associated with it.”
Boam, whose son Joseph is a Reform UK Leicestershire county councillor, said: “I’ve not been happy with things for a little while.”
Smith told the BBC he could not comment and referred the matter to NWLCA chair Keith Merrie.
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