They were fast, omnivorous or herbivorous dinosaurs from the Cretaceous Period of Laurasia (now Asia, Europe and North America), as well as possibly Africa.

The paleontologist Paul Sereno, in 2005, proposed the clade "Ornithomimiformes", defining them as all species closer to Ornithomimus edmontonicus than to Passer domesticus.

[9] Zuolong Tanycolagreus Proceratosaurus Tyrannosauroidea Ornitholestes Compsognathidae Nqwebasaurus Pelecanimimus Shenzhousaurus Harpymimus Beishanlong Garudimimus Deinocheirus Anserimimus Gallimimus Ornithomimus Struthiomimus Alvarezsauroidea Therizinosauroidea Oviraptorosauria Paraves The cladogram below follows the phylogenetic analysis by Scott Hartman and colleagues in 2019, which has included a vast majority of species and uncertain specimens, resulting in a novel phylogenetic arrangement.

Rativates Ornithomimus sedens Anserimimus Qiupalong Dromiceiomimus Struthiomimus Timimus Gallimimus Below is a cladogram by Serrano-Brañas et al., 2020, showing an analysis more in line with previous assumptions about ornithomimosaur classification.

[19] Nqwebasaurus Beishanlong Pelecanimimus Shenzhousaurus Harpymimus Deinocheirus Garudimimus Paraxenisaurus Archaeornithomimus Bissekty Taxon Sinornithomimus Gallimimus Struthiomimus Qiupalong Anserimimus Ornithomimus Ornithomimosaurs probably acquired most of their calories from plants.

Henry Fairfield Osborn suggested that the long, sloth-like "arms" of ornithomimosaurs may have been used to pull down branches on which to feed, an idea supported by further study of their strange, hook-like hands.

[20] The sheer abundance of ornithomimids — they are the most common small dinosaurs in North America — is consistent with the idea that they were plant eaters, as herbivores usually outnumber carnivores in an ecosystem.

Comparisons between the scleral rings of two ornithomimosaur genera (Garudimimus and Ornithomimus) and modern birds and reptiles indicate that they may have been cathemeral, active throughout the day at short intervals.

[22] Multiple specimens of Sinornithomimus were collected from a single monospecific bonebed with a considerable density of juvenile individuals—out of 14, 11 were juveniles—, suggesting a gregarious behavior for an increased protection from predators.

Under this consideration, it is possible that a small pack of more than 10 individuals of different ornithomimosaurian herds was travelling together in optimal areas to find food resources, nesting sites or something else.

[24][25] A right second metatarsal belonging to a large-bodied ornithomimosaur weighing approximately 432 kg has been described from Mississippi with a "butterfly" fragment fracture pattern characteristic of blunt force trauma, likely as a result of an interaction with a predator or a violent bout of intraspecific competition.