Little is known about neighborhood attributes that may influence opportunities for healthy eating and physical activity in relation to breast cancer mortality. We used data from the California Breast Cancer Survivorship Consortium and the California Neighborhoods Data System (CNDS) to examine the neighborhood environment, body mass index, and mortality after breast cancer. We studied 8,995 African American, Asian American, Latina, and non-Latina white women with breast cancer. Residential addresses were linked to the CNDS to characterize neighborhoods. We used multinomial logistic regression to evaluate the associations between neighborhood factors and obesity and Cox proportional hazards regression to examine associations between neighborhood factors and mortality. For Latinas, obesity was associated with more neighborhood crowding [quartile 4 (Q4) vs. Q1: OR, 3.24; 95% confidence interval (CI), 1.50-7.00]; breast cancer-specific mortality was inversely associated with neighborhood businesses (Q4 vs. Q1: HR, 0.46; 95% CI, 0.25-0.85) and positively associated with multifamily housing (Q3 vs. Q1: HR, 1.98; 95% CI, 1.20-3.26). For non-Latina whites, lower neighborhood socioeconomic status (SES) was associated with obesity [quintile 1 (Q1) vs. Q5: OR, 2.52; 95% CI, 1.31-4.84], breast cancer-specific (Q1 vs. Q5: HR, 2.75; 95% CI, 1.47-5.12), and all-cause (Q1 vs. Q5: HR, 1.75; 95% CI, 1.17-2.62) mortality. For Asian Americans, no associations were seen. For African Americans, lower neighborhood SES was associated with lower mortality in a nonlinear fashion. Attributes of the neighborhood environment were associated with obesity and mortality following breast cancer diagnosis, but these associations differed across racial/ethnic groups.