The purpose of this study was to determine if clinical findings, histologic grade, or other histologic features were associated with clinical outcome in dogs with subcutaneous mast cell tumors (MCTs). Medical records of 43 client-owned dogs were retrospectively reviewed, and follow-up information was gathered via phone or follow-up examination. Progression-free survival (PFS), disease-free interval (DFI), and overall survival were calculated. Forty-two and twenty-two dogs, respectively, had grade 2 (Patnaik grading system) or low-grade tumors (two-tier grading system). Median PFS was 1474 days. Median DFI was not reached at >1968 days. Overall median survival time was not reached at >1968 days. In univariate analysis, argyrophilic nucleolar organizer regions (AgNORs), proliferating cell nuclear antigen, and mitotic index were negatively prognostic for PFS whereas Ki-67, proliferating cell nuclear antigen, and microvessel density were negatively prognostic for DFI. In multivariate analysis, AgNORs remained negatively prognostic for PFS. Results suggest that proliferation indices, especially AgNORs, may be useful in predicting the rare poor outcomes in dogs with subcutaneous MCTs. The vast majority of subcutaneous MCTs appear to be low or intermediate grade with excellent outcomes from good local tumor control.