Born in Louisville, Kentucky, Flannery studied at the Art Institute of Chicago, then went into advertising, working his way up to a vice presidency of Young & Rubicam. He retired in 1941 to devote his time to painting and received commissions from prestigious owners and breeders, including Greentree Stables and Alfred G. Vanderbilt. In his one-man shows at New York’s Kraushaar Galleries, he documented American Thoroughbred racing with an insider’s eye.