Background: There are various treatment options for breast cancer, but a delayed diagnosis at an advanced stage negatively affects overall survival. Our objective is to investigate the prognostic significance of the modified Glasgow Prognostic Score (mGPS) with different treatment modalities. Materials and methods: The study was a longitudinal cohort conducted in the Biochemistry laboratory of the Institute of Basic Medical Sciences at Khyber Medical University. Enrolling 121 patients with invasive breast carcinoma from November 2018 to February 2021, blood samples were collected at various stages. Data analysis utilized SPSS^® version 22, employing the Kaplan–Meier method for overall survival (OS) calculation. The log-rank test assessed P-values for OS, and the chi-square test analyzed the association of modified Glasgow Prognostic Score (mGPS) with different treatments. Statistical significance was considered for results with a P-value (p < 0.05). Results: The study results indicated that mGPS 2 was linked to lower overall survival, while mGPS 0 and 1 were associated with good overall survival. Chemo-endocrine therapy exhibited the highest median survival of 40.9 months compared to other treatment options. Conversely, chemoradiotherapy combined with targeted therapy demonstrated the lowest overall median survival of 22.5 months. Conclusion: The findings of the current research suggested that mGPS could be used as a prognostic screening tool in patients with breast cancers receiving different therapeutic modalities.