He was born in New York City! To two British parents. Wikipedia says this:
Johnson’s paternal great-grandfather was Circassian-Turkish journalist Ali Kemal[12][13][14] who was a secular Muslim;
and this:
Johnson’s mother is the granddaughter of Elias Avery Lowe, a palaeographer, who was a Russian Jewish immigrant to the US,[
Very interesting. So Johnson is not a natural born UK citizen but that’s okay by them.