Leo Mielziner was a noted portrait artist, and the son of Dr. Moses Mielziner, Rabbi, and Rosette Mielziner. He was one of ten children, one of whom died in infancy, and had a twin sister, Bella Mielziner. Leo was born in New York City at 22 Jefferson Street, but then moved as a child with his family to settle in Cincinnati, Ohio. His father Moses Mielziner taught at Hebrew Union College and was a noted Talmudic scholar, publishing among other works, An Introduction to the Talmud.

Moses Mielziner married Rosette Levald of Copenhagen in 1861. Of the seven children who survived him, Leo Mielziner was an artist in Paris and Jacob was a rabbi in Helena, Montana. Leo Mielziner would marry Ella Friend McKenna and become the father of five-time Tony Award winning stage designer, Jo Mielziner and of the noted actor and MGM Story Director, Kenneth MacKenna.