Warren was born on February 8, 1950, in Las Cruces, New Mexico, where he grew up, except for attendance of a private school in Evergreen, Colorado, from 1959 through 1962. His father, Anthony, was an accountant, who worked for Porter & Sons and then the First National Bank, while his mother raised four children and worked at Woolworth. Warren attended New Mexico State University from 1968 to 1972, changing his major several times, including math, computer science, and food service management. He worked as a cook and a printer. In 1972 after his father died suddenly, he moved to Albuquerque to attend University of New Mexico as a premedical student with a major in biology. He took a flora of New Mexico class, and through serving as the teaching assistant for the class in the following year, developed a strong interest in botany. He received his BS degree in biology in 1973 and following advice from William Martin began a master’s program in 1974. He was herbarium curatorial assistant and a research assistant on a series of grants. He also ran an independent contracting service for environmental impact and endangered species assessments. His first research projects included: a biological survey of Kirtland Air Force Base; Manual of the Saltbushes (Atriplex) in New Mexico; natural succession on strip-mined lands in northwestern New Mexico; and his thesis on the flora of the Animas Mountains, New Mexico.
In 1977, Warren went to St. Louis to work on a PhD degree with Peter Raven at Washington University and the Missouri Botanical Garden. He worked on systematics of the evening primrose genus, Oenothera, and completed the degree in 1981.
Without ever having visited the Hawaiian Islands or studied island plants, he took a position at the Bishop Museum to work on a new flora of the Hawaiian Islands. He assumed leadership in the project in 1983, and the first flora for over a century was completed in slightly over five years and published in 1990 involving two co-authors and fifty contributors. He subsequently was awarded the Robert Allerton Award in 1995 from the National Tropical Botanical Garden, an award for excellence in the fields of tropical botany or horticulture. He also was awarded the Engler Medal in Silver in 1990 from the International Association for Plant Taxonomy and the Henry Allan Gleason Award from the New York Botanical Garden for his work on the Manual of the flowering plants of Hawaii.
He was hired as curator of Pacific botany, Department of Botany, Smithsonian Institution in 1988 to replace Marie Hélène Sachet and F. Raymond Fosberg. In 1992, he was selected to be department chair and served in this position through 1997. His research primarily focuses on biosystematic, taxonomic and phylogenetic studies, monographs and floras, classification, phylogeny, and biogeography of Pacific island floras contributing to understanding island biodiversity, evolution, and, therefore, conservation. He is the writer or co-writer of 116 publications including four books. His studies continue, including 20 of his publications, in the Onagraceae tribe Onagreae, primarily Oenothera. He has served on the USA board of directors for the International Association of Plant Taxonomists, 1996-99; Systematic Botany Monographs, editorial committee, American Society of Plant Taxonomists, 1995-99; and editor, American Society of Plant Taxonomists newsletter, 1990-92.
In June 1993, he married Lucy Carol Julian in Boulder, Colorado, and they had two children born in Falls Church: Anthony Julian Wagner in 1995 and Eleanor Rose Wagner in 1997.
Warren was elected to the Washington Biologists’ Field Club in 1991 and has served as treasurer from 1997 to 2007. In 1996, he began assisting Tom Fritts on cook duties, and assumed them at the oyster roast in 1998 amidst clouds of smoke (from a squirrel nest in the chimney).