Similar to the modern extant ostriches, emus, and rheas (among other birds), ornithomimid dinosaurs likely lived as opportunistic omnivores, supplementing a largely plant-based diet with a variety of small mammals, reptiles, amphibians, insects, invertebrates, and anything else they could fit into their mouth, as they foraged.

[1] In 1901, Lawrence Lambe found some incomplete remains, holotype CMN 930, and named them Ornithomimus altus, placing them in the same genus as material earlier described by Othniel Charles Marsh in 1890.

However, in 1914, a nearly complete skeleton (AMNH 5339) was discovered by Barnum Brown at the Red Deer River site in Alberta, prompting O. altus to be described as the type genus of a new subgenus, Struthiomimus, by Henry Fairfield Osborn in 1917.

Because dinosaur fauna show rapid turnover, it is likely that these younger Struthiomimus specimens represent a species distinct from S. altus, though no new name has been given to them.

[17] Struthiomimus had a build and skeletal structure typical of ornithomimids, differing from closely related genera like Ornithomimus and Gallimimus in proportions and anatomical details.

[18] As with other ornithomimids, they had small slender heads on long necks (which made up about 40% of the length of the body in front of the hips).

[20] As in other ornithomimids but unusually among theropods, the three fingers were roughly the same length, and the claws were only slightly curved; Henry Fairfield Osborn, describing a skeleton of S. altus in 1917, compared the arm to that of a sloth.

[11] Struthiomimus is a member of the family Ornithomimidae, a group which also includes Anserimimus, Archaeornithomimus, Dromiceiomimus, Gallimimus, Ornithomimus, and Sinornithomimus.

Marsh initially included Struthiomimus in the Ornithopoda, a large clade of dinosaurs not closely related to theropods.

Paul Sereno, for example, used Ornithomimidae to include all ornithomimosaurians in 1998, but subsequently changed to a more exclusive definition (advanced ornithomimosaurs) within Ornithomimosauria,[32] a classification scheme that was adopted by other authors at the beginning of the current century.

[2] This newer view created an image much more reminiscent of modern flightless birds, such as the ostrich to which this dinosaur's name refers, but would only much later be accepted for all theropods.

[3][35] This theory has never been discounted, but Osborn, who described and named the dinosaur, proposed that it probably ate buds and shoots from trees, shrubs and other plants,[18] using its forelimbs to grasp branches and its long neck to enable it accurately to select particular items.

The supposed speed of Struthiomimus was, in fact, its main defense from predators (although it may also have been able to lash out with its hind claws when cornered), such as the dromaeosaurids (e.g. Saurornitholestes and Dromaeosaurus) and tyrannosaurs (e.g. Daspletosaurus and Gorgosaurus), which lived at the same time.

[36] Fossil remains of S. altus are only known definitively from the Oldman Formation, dated to between 78 and 77 million years ago during the Campanian stage of the late Cretaceous period.