Gaining a detailed understanding of the geographic and temporal variations between Lancian mammalian faunas within the Western Interior Basin requires specimens to be associated with precise stratigraphic data, but only a few mammal-producing localities are documented in such detail from the Hell Creek Formation of southwestern North Dakota. Recent work at two sites in southwestern North Dakota resulted in the discovery of additional mammal specimens from previously documented localities for which the stratigraphic position is precisely recorded. The most productive site is situated 12.07 m below the K/Pg boundary and produced 18 mammal specimens that are referable to at least six taxa, two of which (the multituberculate Essonodon browni and the marsupialiform Glasbius twitchelli) are first occurrences for North Dakota. Another important specimen (NDGS 1719) was collected as float approximately 1 m below a documented vertebrate locality that is positioned 2.72 m below the K/Pg boundary. This specimen is also referable to G. twitchelli and consists of a right dentary preserving p2-m4, making it the most complete dentary yet referred to that species. Specimen NDGS 1719 is the stratigraphically highest mammal specimen referred to a specific taxon yet reported from the Hell Creek Formation in North Dakota, and its completeness facilitates a detailed description of the dentition of G. twitchelli. These discoveries expand our knowledge of the Lancian fauna from southwestern North Dakota and demonstrate that further work is needed to ensure that our understanding of the Lancian fauna from North Dakota is comprehensive.