Hardin was born in 1916 in Kansas and then lived in Wyoming, but his family moved to Hurley, Missouri, when he was thirteen. He played in the Hurley High School Band, but when he was sixteen, he lost his sight in an accident when a dynamite cap exploded in his face. He finished high school at a school for the blind in Iowa and later moved to the Batesville, Arkansas, area with his family.
In his mid to late twenties, he moved to New York, made the acquaintance of classical musicians like Leonard Bernstein and jazz performers like Benny Goodman, and soon made his own name in the musical world. In 1947, he started calling himself "Moondog" in honor a dog he had had in Hurley that, according to the musician, howled at the moon more than any dog he had ever heard of.
In 1974, Moondog moved to Germany to be closer to his Viking heritage, and he lived the rest of his life there, dying in 1999.