Despite the important roles small mammals serve in ecosystem processes as prey, predators, and seed and fungus dispersers, small mammals are not widely used to assess the functionality of wildlife crossing structures (WCS). By including the monitoring of small mammals with other target wildlife species, we can assess the quality of the crossing structure across multiple trophic scales. The I-90 Snoqualmie Pass East Project in the Cascade Range of Washington State, USA, is a collaboration among multiple government and non-profit agencies, incorporating long-term monitoring of wildlife connectivity along this critical transportation corridor. Our objective was to compare small mammal species composition and relative abundance pre-construction and post-construction for two adjacent WCS, one overcrossing and one undercrossing. The 150-ft overcrossing was completed in 2018 (vegetation planted 2019), and the 120-ft undercrossing was completed in 2013 (restoration vegetation planted in 2015). Small mammals (mice, voles, shrews, chipmunks) were live-trapped on 60 m x 60 m grids with 49 Sherman traps, 16 Tomahawk traps, and 8 pitfall buckets (in 2 arrays, with drift fencing) in the summers at highway-adjacent forest sites pre- and post-construction (2013, 2014, 2019, 2021) and trapping grids were added within the WCS post-construction (2019, 2021). We identified animals to species, took standard morphological measurements, and applied toe-nail polish as temporary marks. If individuals were large enough, we injected 8- or 11-mm PIT (Passive Integrated Transponder)-tags for mark-recapture and to track individual movements. Adjacent forests were inhabited by a variety of small mammal species, including shrews, shrew-moles, deer mice, voles, jumping mice, chipmunks, and flying squirrels. The number of forest species trapped on the crossing structures increased between 2019 and 2021, not yet including the full composition of the forest small mammal community nearby. Multiple individuals from several species moved from forest into the crossing structure, and a few individuals were tracked moving all the way across the highway within a several-week period in 2021. Movement within crossing structures, as well as presence of different age classes and reproductive adults suggest that these crossing structures provide not only safe passage across the highway but also viable habitat for home ranges for small mammals. Future monitoring of these sites will allow us to quantify how many years it may take for all (or most) forest species to eventually discover and use the WCS for movement across the roadway, and for more detailed data on individual movement, which has not been well described for many small mammal species. We provide suggestions for monitoring of small mammals on other wildlife crossing structure projects, even those not specifically designed for use by a full spectrum of species.