Background and Objectives: Cancer survival poses significant challenges in oncology, with lifestyle modifications increasingly recognized as crucial in modifying patient outcomes post-diagnosis. This meta-analysis aims to systematically evaluate the impact of various lifestyle interventions on cancer survival across different types of cancer. Methods: A comprehensive literature search of electronic databases including PubMed, Scopus and Cochrane was performed to identify relevant studies up to 30 November 2024. Relevant studies were chosen and data were extracted and analyzed using SPSS Version 29.0 software. Results: Our systematic review included data from 98 studies involving a total of 1,461,834 cancer patients to evaluate the impact of lifestyle factors on cancer survival. Out of these, 64 studies were included in the meta-analysis. Our meta-analysis demonstrates that adherence to specific dietary patterns significantly improves cancer-specific outcomes. The Healthy Eating Index (HEI) diet was associated with a reduction in cancer-specific mortality (pooled log HR: −0.22; 95% CI: [−0.32, −0.12]; p < 0.001). Similar benefits were observed with the Mediterranean diet (aMED), which also reduced cancer mortality and recurrence (pooled log HR: −0.24; 95% CI: [−0.40, −0.07]; p < 0.001), and the Dietary Approaches to Stop Hypertension (DASH) diet (pooled log HR: −0.22; 95% CI: [−0.33, −0.12]; p < 0.001). Additionally, general dietary improvements were beneficial for breast cancer-specific mortality across 17 cohort studies (pooled log HR: −0.15; 95% CI: [−0.25, −0.06]; p < 0.001). Engaging in any form of physical activity post-diagnosis was associated with significant improvements in cancer-specific mortality or recurrence (pooled log HR: −0.31; 95% CI: [−0.38, −0.25]; p < 0.001). Participants who ceased smoking after diagnosis exhibited more favorable cancer outcomes (pooled log HR: −0.33; 95% CI: [−0.42, −0.24]; p < 0.001), with smoking cessation notably reducing cancer-specific mortality among lung cancer survivors (pooled log HR: −0.34; 95% CI: [−0.48, −0.20]; p < 0.001). Additionally, reducing alcohol intake post-diagnosis significantly improved cancer outcomes (pooled log HR: −0.26; 95% CI: [−0.33, −0.19]; p < 0.001). Alcohol moderation in gastrointestinal tract cancer survivors specifically decreased both cancer-specific mortality and recurrence (pooled log HR: −0.22; 95% CI: [−0.29, −0.15]; p < 0.001). Conclusions: Lifestyle modifications after cancer diagnosis significantly improve cancer-specific outcomes. Specific dietary patterns, increased physical activity, smoking cessation, and reduced alcohol intake are all associated with lower cancer-specific mortality. Integrating these lifestyle changes into oncology care may enhance patient survival and quality of life.