Necromys benefactus was described from a specimen collected at Bonifacio in southwestern Buenos Aires Province. A central Argentine study recorded captures along weedy margins of crop fields and roads.
The 1919 description gives a back mixing black and dull buff-brown, and grey underparts with slaty hair bases and greyish-white tips. The adult male type measured 99 mm in head-and-body length and 65 mm in tail length.
A 2005 study trapped specimens treated as N. benefactus at Oliveros and Uranga in Santa Fe Province and Pergamino in Buenos Aires Province, using Sherman traps along weedy margins of crop fields and roads. These are capture-site records, not observations of diet or everyday behavior.
An extinction-risk assessment has not been added yet.