Municipality Mirna, which is part of the Jugovzhodna Slovenija statistical region, measures 31 km2; this ranks it 176th among Slovene municipalities.

Statistical data for 2020 reveal the following:

In the middle of 2020 about 2,690 people (about 1,360 men and 1,330 women) were living in the municipality, which ranks Mirna 162nd among Slovene municipalities. The population density was 86 people per square kilometre, which was lower than the national average of 104 people per square kilometre.

The number of live births was lower than the number of deaths, which means that natural increase per 1,000 population in the municipality was negative. It was -0.4 (in Slovenia -2.5). The number of people who moved from the municipality was lower than the number of people who moved into the municipality, so net migration per 1,000 population in the municipality was positive. It was 12.6. The sum of natural increase and net migration per 1,000 population in the municipality was positive. It was 12.3 (in Slovenia 6.2).

The mean age of people in Mirna was 41.9 years, which was lower than the national average (43.6).

As in most Slovene municipalities, in Mirna the number of old people was higher than the number of young people: there were 101 people, aged 65 or more, per 100 people aged 0–14. This ratio shows that the value of the ageing index was lower than the national average of 136. It also shows that the mean age of people in Mirna is growing more slowly than in Slovenia as a whole. Data by sex show that the ageing index for women was in all municipalities except in three (Dobrovnik/Dobronak, Jezersko in Loški Potok) higher than the ageing index for men. In Mirna – as in most Slovene municipalities – more women were 65+ years old than 14 or less; for men the situation was exactly the opposite.

Mirna had 1 kindergarten, which was attended by 132 children. Of all children in the municipality aged 1–5, 100% were included in kindergartens, which was more than national average of 81%. In the school year 2020/21 elementary schools in Mirna were attended by about 290 pupils, while various upper secondary schools were attended by about 70 pupils. There were 42 students and 8 graduates per 1,000 people in the municipality; in Slovenia the averages were 39 per 1,000 and 7 per 1,000, respectively.

Among people aged 15–64 (i.e. working age population) about 72% were persons in employment (i.e. persons in paid employment or self-employed persons), which is more than the national average (66%).

In Mirna, average monthly gross earnings per person employed by legal persons were about 15% lower than the annual average of monthly earnings for Slovenia; net earnings were about 12% lower.

Among 1,000 inhabitants in the municipality, 587 had a passenger car. The car was on average 10 years old.

In 2020, 266 kg of municipal waste per person was collected in Mirna, which is 89 kg less than on average in Slovenia.