The ability of a crop species to delay senescence or “stay green” under abiotic stress conditions during the grain fill period has been associated with increased productivity. Functional stay-green describes plants that can continue photosynthesis late into the growing season. Natural variation for the stay-green trait exists in the germplasm of maize, particularly tropical maize, but little is known of the inheritance, expression, and physiology of this trait. A germplasm screening and gene discovery project was initiated in diverse germplasm sources to address questions about the potential for using the stay-green trait to improve performance of maize. Germplasm screening experiments demonstrated that Mo20W expressed functional stay-green under abiotic stress conditions during late grain fill. Mapping populations were created by crossing Mo20W with B73 and Mo17 to produce backcross (BC2) mapping populations with B73 and Mo17 as the recurrent parents. Relative greenness and chlorophyll fluorescence were used to determine visual stay-green and functional stay-green, respectively. Results from experiments in Garden City, KS showed significant variation for visual and functional stay-green within each mapping population at approximately 1000 growing degrees after silking. Contrasting groups of stay-green and senescent progeny were evaluated for genetic differences in single nucleotide polymorphism (SNP) markers. Single factor and bulked-segregant analyses indicated four major genetic loci linked with the functional stay-green phenotype. Testcross hybrids of the Nested Association Mapping (NAM) populations of maize were similarly evaluated for variation in expression of stay-green and discovery of quantitative trait loci (QTL) associated with this trait. The populations of testcross hybrids were evaluated in NC, IN, IA, and MO. Plant phenotyping experiments showed significant variation in expression of stay-green, and joint-QTL analysis identified two additional QTL for stay-green. The favorable donor alleles for these QTL were discovered in exotic genetic backgrounds.