The first remains were found in 1997, in the Late Cretaceous strata of the Ulansuhai Formation located at Alshanzuo Banner, Inner Mongolia Autonomous Region, Northern China.

The first fossil remains of Sinornithomimus were uncovered by Dong Zhiming in the Ulansuhai Formation as part of the Mongol Highland International Dinosaur Project in 1997.

The unweathered state of the bones, preserved in siltstone interspersed with layers of clay and the absence of evidence for post-mortem movement, argue for a catastrophic event that killed all the individuals present in the find simultaneously and instantaneously.

[3] The following cladogram follows that of Xu and colleagues in 2011:[6] Archaeornithomimus Sinornithomimus Anserimimus Gallimimus Qiupalong Struthiomimus Ornithomimus The herbivory of this genus is supported by the rare presence of gastroliths, as such stones were found forming conspicuous masses in the stomach areas of the fossil skeletons.

This is furthermore supported by the increase of running ability as the animal progressed in its ontogeny, thus shown in the more cursorial proportions of the adults which possess relatively longer lower legs.