Background: Many studies explored the prognostic value of the modified Glasgow Prognostic Score (mGPS) in urothelial carcinoma (UC), but the results are controversial. This study aimed to quantify the relationship between pretreatment mGPS and survival in patients with UC. Methods: A systematic literature search was conducted using Embase, PubMed, and Web of Science to identify eligible studies published before August 2022. Pooled hazard ratios (HRs) with 95% confidence intervals (CIs) were used to assess the association between pretreatment mGPS and the prognosis of UC. Results: Thirteen eligible studies involving 12,524 patients were included. A high mGPS was significantly associated with poor overall survival (mGPS 1/0: HR = 1.33, 95% CI 1.12–1.58, p = 0.001; mGPS 2/0: HR = 2.02, 95% CI 1.43–2.84, p < 0.0001), progression-free survival (mGPS 1/0: HR = 1.26, 95% CI 1.03–1.53, p = 0.021; mGPS 2/0: HR = 1.76, 95% CI 1.12–2.77, p = 0.013), recurrence-free survival (mGPS 1/0: HR = 1.36, 95% CI 1.18–1.56, p < 0.0001; mGPS 2/0: HR = 1.70, 95% CI 1.44–2.000, p < 0.0001), and cancer-specific survival (mGPS 2/0: HR = 1.81, 95% CI 1.30–2.52, p < 0.0001). A subgroup analysis of OS also yielded similar results. Conclusions: Evidence suggests that high pretreatment mGPS in UC is closely related to poor survival. Pre-treatment mGPS is a powerful independent prognostic factor in patients with UC.