He earned his PhD from the University of Colorado at Denver in 1976 and then he moved to Colombia. Defler worked in eastern Colombia, in the Llanos until 1984 and then in the Amazonian Vaupés Department where he developed and lived in his research station, Estación Biológica Caparú until 1998 when he was obligated to flee from his research station by FARC guerrillas. He had run a primate rehabilitation center in Vaupes. He is the author of many papers about primates and of the books Primates de Colombia (2003), Primates of Colombia (2004) and Historia Natural de los Primates Colombianos (2010) .
Currently, he heads another Amazonian research station that he has developed in the southern Colombian Amazon, Estación Ecológica Omé, that is affiliated with the National University of Colombia and he teaches at the Bogotá campus of the same university. He and the Colombian biologist Marta Bueno are credited with first describing Hernández-Camacho's night monkey (Aotus jorgehernandezi) in 2007. Together with Javier Garcia, he led an expedition in which they discovered and described a new species of titi monkey, the Caquetá titi (Callicebus caquetensis). Using karyotypes, Defler has done work clarifying the taxonomy of various species of night monkey (Aotus). He has done field studies in the Colombian llanos and the Colombian Amazon on the white-fronted capuchin (Cebus albifrons), the brown woolly monkey (Lagothrix lagothricha), the black-headed uakari (Cacajao melanocephalus), the black titi (Callicebus lugens), the Lucifer titi (Callicebus lucifer), and the Venezuelan red howler (Alouatta seniculus) and has accomplished many primate censuses in different parts of eastern Colombia.
- "Thomas R. Defler". Retrieved 2012-08-17.
- Defler, T. (2004). Primates of Colombia. Conservation International. ISBN 1-881173-83-6.
- Semple, K. (2000). A habitat held hostage.
- "In Colombia, activist works to preserve monkeys". Los Angeles Times. July 10, 2010. Retrieved 2012-08-17.
- Defler, Thomas R.; Bueno, Marta L. (2008). "Aotus diversity and the species problem". Primate Conservation 22 (1): 55. doi:10.1896/052.022.0104. 136211.
- Defler, Thomas R.; Bueno, Marta; Garcia, Javier (2010). "Callicebus caquetensis: A new and critically endangered titi monkey from Southern Caquetá, Colombia". Primate Conservation. 136211.
- "Newly discovered monkey nearly extinct". NTDTV. Retrieved 2012-08-17.
- Moseman, A. (August 13, 2010). "Rare Redbearded Monkeys Found Living in Colombia". CBS News. Retrieved 2012-08-17.
- Defler, T. (2004). Primates of Colombia. Conservation International. pp. 252–266. ISBN 1-881173-83-6.