The coastal upwelling ecosystem near Monterey Bay, California, is a productive yet variable ecosystem and an important foraging area for many mobile apex predators, such as marine mammals. Long-term studies are necessary to better understand how wide-ranging predators respond to temporal environmental variability; however, few of these studies exist. We conducted monthly shipboard line-transect surveys in Monterey Bay from 1997 to 2007. We identified 22 species of marine mammals, and calculated monthly and annual densities for the 12 most commonly sighted (focal) species. Species richness remained relatively constant (mean richness ± SE: 13.7 ± 0.396 species yr ) during the 1997/1998 El Niño event. There were no statistically significant differences in the densities of marine mammal species between warmer and cooler years. The community and species-specific responses of marine mammals to warm-water years differed depending on the mechanism of oceanographic variability. During the 1997/1998 El Niño (a basin-wide event), marine mammals aggregated in nearshore areas, such as Monterey Bay, with relatively greater productivity than offshore regions, whereas during anomalous upwelling conditions of 2005 (a more localized oceanographic event), marine mammals redistributed away from Monterey Bay to areas less affected by the anomaly.