The aim of this study was to examine the associations among mentoring relationship quality (i.e., relational and instrumental quality), racial discrimination and coping efficacy with racial discrimination. Three social support models were tested, including the stress buffering, support mobilization, and support deterioration models. Participants were 257 urban, low-income Latina/o high school students, who completed surveys in both 9th and 10th grades. While controlling for gender and coping efficacy with discrimination in 9th grade, results supported the social support deterioration model. Specifically, there was a significant indirect effect of racial discrimination in 9th grade on coping efficacy in 10th grade through instrumental mentoring quality. As racial discrimination increased, mentoring quality decreased and then coping efficacy decreased. We also found that more racial discrimination in 9th grade was significantly associated with lower coping efficacy in 10th grade, and higher instrumental mentoring quality in 9th grade was significantly associated with higher coping efficacy in 10th grade, while controlling for gender and coping efficacy in 9th grade. Implications and recommendations for future research are discussed.