Mechanical thinning is an important silviculture technique for timber production and reducing woody fuels in coniferous forest, but little is known about its effect on wildlife in mixed-coniferous forests in the American Southwest. During 2005–2006 we examined diversity, abundance and survival of terrestrial mammals in thinned and non-thinned mixed-coniferous forest in the Sacramento Mountains, Lincoln National Forest, in southern New Mexico. The three thinning treatments included two non-commercial thins with different slash treatments (i.e., lop-pile, lop-scatter) and a commercial harvest using selective logging. There were two non-thinned treatments that differed in age of stand (i.e., 20–30 years and 60–100 years post harvest). In general, thinned treatments had higher richness and abundance of mammals in comparison with the older non-thinned stand, but did not differ in richness and abundance from the younger non-thinned stand. Abundance of the North American deermouse (Peromyscus maniculatus) and red squirrel (Tamiasciurus hudsonicus) did not differ among treatments. However, survival of P. maniculatus varied more in the non-thinned stands. Abundances of the gray-footed chipmunk (Tamias canipes) and long-tailed vole (Microtus longicaudus) were significantly lower in the older non-thinned stand than in all thinned treatments. Most large mammals were documented in thinned treatments. These results suggest that thinning older stands of mixed-coniferous forest that are overly dense compared with historical conditions benefit the mammal community through increases in diversity and abundance. In comparing the three mechanical-thinning treatments evaluated, none provided a clear cost or advantage to mammals. However, additional controlled experiments are needed to further corroborate these results.