He went by "Johnny" ("Jancsi" in Hungarian) his entire life.
Here's Freeman Dyson calling him "Johnny" throughout a talk Dyson gave at Brown University: https://www.ams.org/notices/201302/rnoti-p154.pdf
Here's Freeman Dyson calling him "Johnny" throughout a talk Dyson gave at Brown University: https://www.ams.org/notices/201302/rnoti-p154.pdf
That is, was "Johnny" something his friends called him, or what he called himself in general when introducing himself to people? I'm getting the impression it was more the latter.