Robert M. Mengel was born in 1921 in Glenview, Kentucky. As a young child, Mengel was interested in nature and birds, writing and illustrating his own bird books.

Mengel served in the U.S. Army Air Force from 1942 to 1946, serving in the Army Airways Communications System overseas from April 1944 to December 1945.

Mengel received a B.S. from Cornell University in 1947. He went on to receive his M.A. and Ph.D. degrees from the University of Michigan. His research interests were the systematics, ecology, biogeography, and evolution of birds and mammals.

His first work at the Museum of Natural History at the University of Kansas was a painting of a flock of geese on the dome of the Panorama of North American Plants and Animals. He was a research associate at the museum from 1953 until he became curator of ornithology in 1967.

He was a Fellow of the American Ornithologists' Union and edited The Auk, journal of the Aou, from 1963 to 1967. From 1970 to 1974, he edited the Aou Ornithological Monographs. He wrote The Birds of Kentucky in that series and more than one hundred scientific papers.

Mengel was also an artist and illustrator, working primarily in watercolor and pen and ink. Among the publications he illustrated, in part or whole, were The Birds of Kentucky and Birds of Colorado, published by the Denver Museum of Natural History, and the five-volume Handbook of North American Birds, published by Yale University Press and sponsored by the Aou and the Smithsonian Institution. In 1988 he was one of eleven U.S. artists, among forty-three worldwide artists, included in Twentieth Century Wildlife Artists.

After Mengel's death, his wife continued to work to have his manuscript, then called You Should Have Been Here Last Week, published. Finally in 1993, Mengel's book, Fly Fisherman's Odyssey, was published by Lyons and Burford.