Abstract Background: Lifestyle habits can impact breast cancer development, but its impact on breast cancer prognosis remains unclear. We investigated associations of post-diagnosis lifestyle with mortality and recurrence in 1,964 women with invasive breast cancer enrolled in the Kaiser Permanente Northern California Pathways Study shortly after diagnosis with lifestyle information at baseline (2005–2013) and the 2-year follow-up. Methods: We calculated a post-diagnosis lifestyle score (range, 0–18) based on 9 diet, physical activity, and body weight recommendations from the American Cancer Society/American Society of Clinical Oncology (ACS/ASCO) using follow-up data (body weight also included baseline data); higher scores indicate greater guideline concordance. Similarly, we calculated a pre-diagnosis lifestyle score using baseline data to investigate pre- to post-diagnosis changes. We estimated hazard ratios (HR) and 95% confidence intervals (CI) using Cox proportional hazard models, with follow-up through December 2018 (observing 290 deaths and 176 recurrences). Results: The 2-year post-diagnosis lifestyle score was inversely associated with all-cause mortality (ACM; HR per 2-point increase = 0.90; 95% CI, 0.82–0.98), and breast cancer–related mortality (HR, 0.79; 95% CI, 0.67–0.95), but not recurrence. Relative to women who maintained low concordance with recommendations at both time points, women who maintained high concordance had a lower risk of ACM (HR, 0.61, 95% CI, 0.37–1.03). Improved concordance with some specific recommendations (particularly PA) may be associated with a lower hazard of ACM (HRPA, 0.52; 95% CI, 0.35–0.78). Conclusions: Results suggest that women with breast cancer may benefit from a post-diagnosis lifestyle aligned with ACS/ASCO guidelines. Impact: This information may potentially guide lifestyle recommendations for breast cancer survivors to reduce mortality risk.