A method of forming a gate cut isolation, a related structure and IC are disclosed. The method forms a dummy gate material mandrel having a sidewall positioned between and spaced from a first active region covered by the mandrel and a second active region not covered by the mandrel. A gate cut dielectric layer is formed against the sidewall of the mandrel, and may be trimmed. A dummy gate material may deposited to encase the remaining gate cut dielectric layer. Subsequent dummy gate formation and replacement metal gate processing forms a gate conductor with the gate cut isolation electrically isolating respective first and second portions of the gate conductor. The method creates a very thin, slightly non-vertical gate cut isolation, and eliminates the need to define a gate cut critical dimension or fill a small gate cut opening.