Completeness of mast cell tumour (MCT) excision is determined by assessment of histologically tumour-free margins (HTFM). The HTFM width necessary to prevent local recurrence (LR), recognized as histologic safety margin (HSM) in human oncology, has not been defined. We hypothesized that HTFM width would correlate with risk for LR and high-grade tumours would require wider HTFM than low-grade tumours. Records of dogs with completely excised MCTs were included. Signalment, two-tier tumour grade, tumour size, HTFM width, recurrence and therapy data was collected. High-grade (n = 39) tumours were more likely to recur than low-grade (n = 51) tumours (35.9% versus 3.9%), P < 0.0001, with no association between HTFM width and LR. Twenty-nine percent of low-grade tumours had HTFM less than 3 mm; none recurred. Narrow (≤3 mm) histologic margins are likely adequate to prevent LR of low-grade tumours. High-grade tumours have significant risk of LR regardless of HTFM width.