There are two main types of residents in Marne City: those under the age of 18 and those over the age of 65. The age of the population was calculated by taking the median age of the city's residents, which was 40.2. There were approximately 8.8% underage people and 17.4% of adults were over the age of 65. The gender makeup of the city was 48.2% males to 51.8% females.
The population of Marine City is approximately 4,500 people, which is well below the state and county averages. The majority of residents, who lived in the town between 1900 and 2000, were married couples, although nearly a third of the households had a female householder without her husband present. The rest of the population lived in one of the city's four apartment buildings, and just under half of the households were family units. About 11.6% of households had a single female householder, and 30.4% of households had a senior citizen. The average household size in the city was 2.50 people.
The city's population is continually changing, but it has remained fairly stable over the past few decades. It had 3,462 residents in 1930, a 7.2% decrease from 1920 to 1930. The city lost 269 residents during this time. Between 2010 and 2020, it lost 10.3% of its population, or 573 people. It gained 637 residents between 1940 and 1950, or 17.5% of its entire population.