The majority of Mahakam Ulu's population was Catholic in June 2021.

The Directorate General of Population and Civil Registration (Dukcapil) of the Ministry of Home Affairs recorded that the population of Mahakam Ulu Regency, East Kalimantan, was 135,171 people in June 2021. Of this number, the majority, or 21,692 people (61.68%), are Catholic.

There are also 8,230 people (23.4%) in Mahakam Ulu Regency who are Muslim, 5,174 people (14.71%) are Christian, and 70 people (0.2%) are Hindu. There are also 3 people (0.01%) adhering to indigenous beliefs, and 2 people (0.01%) are Buddhist.

Based on gender, 18,772 people (53.37%) of the population of Mahakam Ulu Regency are male. There are also 16,399 people (46.63%) who are female.

According to marital status, 17,611 people (50.07%) of the population of Mahakam Ulu Regency are unmarried, and 15,959 people (45.38%) are married. There are also 467 people (1.33%) who are divorced, and 1,134 people (3.22%) are widowed.

Mahakam Ulu is one of the regencies in East Kalimantan with an area of 15,315 square kilometers and a population density of 2 people per square kilometer. Administratively, the regency is divided into 5 sub-districts consisting of 50 villages.