Risk of exposure to Borrelia burgdorferi sensu lato (s.l.) spirochetes, which include the causative agents of Lyme disease, is, in part, determined by the density of questing infected vector ticks. We sought to clarify the temporal patterns of nymphal activity, and the extent of variation in peak and cumulative densities of B. burgdorferi s.l.-infected Ixodes pacificus Cooley & Kohls nymphs, at 12 sites within the ecologically diverse Mendocino County in northwestern California. Also, we assessed the impact of various environmental characteristics (e.g., climatologic variables, habitat type, deer usage) on the aforementioned tick-related traits. The average durations of total and peak (nymphal density > 75% of absolute peak) questing activity were 31% and 82% longer, respectively, in areas with conifers present than in oak woodlands, which represented the warmest and driest habitat type examined. Peak and cumulative densities of infected nymphs varied > 400-fold between sites. Both traits were positively associated with the presence of Quercus spp. oaks or deer, and lower in redwood/tanoak versus oak and oak/Douglas fir habitats. However, a prolonged duration of nymphal activity in redwood habitats, relative to oak woodlands, resulted in a shift from peak nymphal densities occurring in oak woodlands in spring to redwood/tanoak habitats in summer. In conclusion, our data clearly show significant variability in seasonal as well as spatial risk of exposure to Lyme disease spirochetes within a small but ecologically, diverse geographic area. Hence, temporally dynamic and spatially explicit models are needed to assess the risk of exposure to tick-borne pathogens at spatial scales encompassing diverse climatologic or ecological conditions.