Background Canine mammary tumors (CMTs) are among the most frequently diagnosed neoplasms in female dogs and represent an important health concern in companion animal medicine. However, their overall contribution to the canine tumor spectrum in mainland China has not been systematically quantified.Results A systematic search of CNKI, Wanfang, VIP, PubMed, and Web of Science Core Collection was conducted from database inception to March 2025 to identify studies reporting the number of CMTs and total canine tumors in mainland China. Pooled proportions were estimated using a random-effects model with Freeman–Tukey double arcsine transformation. Thirty-one studies involving 5,947 canine tumor cases were included. The pooled proportion of CMTs among all canine tumors was 34.0% (95% CI: 29.6%–45.8%), with substantial heterogeneity (I² = 91.0%). Higher proportions were observed in female dogs, intact dogs, small-sized breeds, and dogs aged ≥ 6 years. Regional variation was evident, with the highest proportion reported in Northeastern China, and temporal differences were also observed across study periods. No significant publication bias was detected, and sensitivity analysis confirmed the robustness of the results.Conclusions CMTs represent a substantial component of the canine tumor spectrum in mainland China. The observed demographic patterns support the relevance of spontaneous CMTs as a comparative model for human breast cancer research.