Well, to make a long story short, Josh Radnor is Jewish!
Since Josh Radnor's mother is Jewish, and in Judaism one's religion is based on one's mother's religion, then that makes him also Jewish. His father is Jewish too.
Josh Radnor (whose full birth name is Joshua Thomas Radnor) was born on July 29, 1974 in Columbus, Ohio.
Radnor was raised in Conservative Judaism. and attended the Orthodox Jewish day school Columbus Torah Academy. In 1997, Radnor participated in an Israel experience program in Tzfat with Livnot U'Lehibanot.
Radnor is best known for portraying Ted Mosby on the popular and Emmy Award–winning CBS sitcom How I Met Your Mother.
If you see any details that require update, please Let us know!