Nelson Leonard worked with teams of chemists on the synthesis and production of Chloroquine in time for the antimalarial drug to be used in the Pacific theatre of World War II. At the University of Illinois, Urbana-Champaign, and the California Institute of Technology, he became a world leader in organic chemistry and helped found the field of bioorganic chemistry.

Leonard investigated the organic compounds that initiate plant, flower, and tree growth from tissue culture, which is vital to horticulture and agriculture. His group helped make fluorescent derivatives of nucleic acid bases to establish their location in enzymes and proteins. His techniques for derivatization of nucleosides, nucleotides and coenzymes and the preparation of fluorescent probes and artificial DNA were widely cited.

From 1943 until 1955, when he was elected to the National Academy of Sciences, Leonard made solo appearances as a bass-baritone with the Chicago, Cleveland and St. Louis Symphony Orchestras.