The population of critically endangered white-thighed colobus monkeys (Colobus vellerosus) at Boabeng-Fiema Monkey Sanctuary (BFMS) is possibly the only growing population of this species in West Africa. We assessed the current population status of C. vellerosus in BFMS and the surrounding fragments in Ghana. We undertook a complete count of the population in 2020, and this data was combined with previously conducted complete counts from 1990 to 2014. Results show that the total population growth rate of colobus monkeys at BFMS and the surrounding forest fragments was 353.9% between the 1990 and 2020 censuses (at a rate of 11.8% annually). In the BFMS alone, the total population growth rate was 252.3% between 1990 and 2020 (i.e., at a rate of 8.4% annually). The total population growth rate in the surrounding forest fragments was 97.0% between the first census year of 1997 and the 2020 census (i.e., at a rate of 4.2% annually). The mean group size in the BFMS was 16.7 individuals (SD = 4.0; range = 9–25), while that of the surrounding forest fragments was 14.4 individuals (SD = 4.6; range = 9–23). The overall mean group size was 16.1 individuals (SD = 4.3; range = 9–25). An approximate ratio of one adult male to three adult females (1:3.4) and one adult female to one immature (1:1.2) is an indication that the population of C. vellerosus still has the potential to increase further when new suitable forest fragments are explored in the future. C. vellerosus has the potential to increase further in population in small, suitable fragments if habitat destruction and settlement expansion are managed with primate conservation intentions.