Species of Monomorium are very diverse in size and habits, ranging from very small generalist scavengers to large, polymorphic seed harvesters.  It is especially diverse in the Old World with only a handful of species in North America, some of them tramps introduced from other areas. Nests vary from small colonies under rocks to large, low mounds, and a few species nest arboreally under bark or in rotten wood either on trees or on the ground.  Many species form small mounds in open soil which are highly visible because of the large number of workers foraging in its vicinity.  Colonies contain multiple queens (are polgynous) and often consist of a series of individual nests (are polydomous). A few species are inquilines in the nests of related species while others have wingless (apterous) queens or have both wingless and winged (alate) queens. Foraging is primarily during the day, with some species limited to the hotter periods.  Some species, especially those introduced  through human activity, can be significant pests as they forage in houses and buildings.
Monomorium is also known from Oligocene fossils (35-25 million years before present).