As per the Indian Census report of 2001, Mariahu had a population of 20, 142. To be specific, males account for 53% of the population and females comprises of the remaining 47%. In Mariahu, 17% of the population is less than six years of age. The Census report also throws light on the literacy rate of the town, which gives an idea about the status of the population. The average literacy rate of Mariahu is 59%, which is slightly lower than the national average literacy rate, which is 59.5%. Out of it, the male literacy rate is 67% and the female literacy rate is 50%.
Trilochan Mahadev Temple is a major landmark of the town.