Gilliard was born in York, Pennsylvania. He began a lifelong association with the American Museum of Natural History (AMNH) in New York in 1932 by working there as a volunteer. In 1933 he was employed there as an assistant and subsequently passed through the full range of promotions to become Curator of Birds in 1963, unexpectedly dying in office two years later, in his 53rd year, of a sudden heart attack.
