Mammals that cope with harsh environmental conditions by hibernating show seasonal variation in body mass, as fat reserves are accumulated during summer and depleted during winter. In hibernating species, pre-hibernation mass is often considered a key phenotypic trait that can influence future survival and reproductive success—yet we know little about how variation in environmental conditions and seasonality affect pre-hibernation mass. We used 30 years of demographic data from a high-elevation population of golden-mantled ground squirrels (Callospermophilus lateralis) to identify life history and environmental variables that influenced pre-hibernation mass in females. Additionally, we determined if pre-hibernation mass influenced overwinter survival or subsequent reproduction. Growth curves revealed female-biased size dimorphism in body mass; adult mass was not attained until age 2 years, so we considered 3 age classes of females: juveniles, yearlings, and adults (≥2 years). We found that earlier snow melt during spring is associated with higher subsequent pre-hibernation body mass for yearlings and adults, and that juveniles emerging earlier from their natal burrows had higher pre-hibernation masses than later-emerging juveniles. Estimates of food quantity or quality did not affect pre-hibernation mass for any age class, nor was there an effect of reproductive status. Juvenile females with greater pre-hibernation mass were more likely to survive overwinter and reproduce as yearlings, but we found no such effect for yearlings or adult females. We did not find an effect of snow depth or winter duration on either survival or reproduction. Our results indicate that time appears to be the key variable for golden-mantled ground squirrels to acquire sufficient body mass to survive and reproduce, but the effects vary by age class.