 He formerly served as Vice Minister of Agriculture (1989–1993) and Vice Governor of Phnom Penh (1980–1989).Prior to these positions he served as vice president of the Phnom Penh party committee (1981–1989).A natural scientist by training, Mareth earned a bachelor's degree in agronomy and fisheries engineering from the University of Agronomy, Phnom Penh, Cambodia in 1970 and his PhD in animal and aquatic biology from Paul Sabatier University, in Toulouse, France in 1974.