No larger than a small bobcat, it is the smallest known saber-toothed mammal currently recognized by science.

Nanosmilus was first discovered in 1880, by Edward Drinker Cope, and described from fragmentary material.

Its similarities to Eusmilus are such that they were often considered to be members of the same genus as recently as 2013; however, a 2016 phylogenetic analysis found it to be a separate taxon.

Its skull is narrower than both Hoplophoneus and Eusmilus and its sagittal crest, which is smaller, separates into a "V" shape above the glenoid facet, which is different to the sagittal crest's divergence points in Eusmilus and Hoplophoneus respectively.

Unlike Eusmilus, Nanosmilus was equipped with smaller flanges on its lower jaws than its later relative Eusmilus had to protect its saber-teeth, and it likely was more vulnerable to tooth breakage due to the lack of this protective adaptation.