The mona monkey (Cercopithecus mona) is the major non-human primate adapted to swamp forests that characterizes Lagos State. It is useful in ecological services as seed disperser and reforestation. Efforts to legally conserve this species have been ineffective due to non-enforcement of conservation laws. This study determined the population estimate and threats to C. mona in order to provide useful information for its conservation in Lagos State. Using total head count and questionnaires, the occurrence, abundance, population composition, and the anthropogenic threats to the C. mona in three Local Government Areas (LGAs) viz: Eti-Osa, Ibeju-Lekki and Kosofe were determined. Population surveys were conducted during the wet and dry seasons from September, 2021 to April, 2022. Data were collected through field surveys and administration of questionnaires. The data were analysed descriptively and inferentially. Significant differences were at p<0.05. Mona monkeys were sighted in all the three LGAs. The estimated population was 466 individuals. The highest wet and dry season’s population of 206 and 196, respectively were recorded in Ibeju-Lekki LGA. The juveniles (185) and adult females (104) were the most abundant in the three LGAs. There was a significant difference (Chi-square = 16.18, p< 0.05) in the population of infants in Ibeju-Lekki and Kosofe LGAs. A decline in C. mona population was indicated by 54.4% of the respondents, while 50.8% showed that high rate of urbanization was a major threat to the monkeys. Most respondents (74.7%) indicated that decline in forest cover over the years was affecting the monkeys’ population. The need for conservation of C. mona is critical in Lagos State. This could be achieved through the effective enforcement of existing conservation laws and promulgation of new efficient ones.