Glucocorticoid administration is a common clinical practice in attempt to decrease inflammation associated with, and improve resectability of, canine mast cell tumors (MCTs). However, the impact of neoadjuvant glucocorticoids on histological features and proliferation indices of canine MCTs is unknown. The objective of this study was to evaluate changes in tumor grade, mitotic count, Ki67, AgNOR, and AgNORxKi67 scores following short-course, anti-inflammatory neoadjuvant prednisone in canine patients with MCTs. This was a prospective, single-arm pilot study. Client-owned dogs with treatment-naïve, cytologically-confirmed MCTs were enrolled. Patients underwent an initial incisional biopsy followed by a 10-14 day course of anti-inflammatory prednisone, and surgical resection. All histological samples were randomized, masked, and evaluated by a single pathologist. Unstained paired pre- and post-treatment samples were submitted to a commercial laboratory for Ki67 and AgNOR immunohistochemical analysis. There were 11 dogs enrolled with 11 tumors. There were no statistical differences between the pre- and post-treatment histological parameters of mitotic index, Ki67, AgNOR, or Ki67xAgNOR. There were no clinically-significant alterations between pre-treatment and post-treatment in the assignment of tumor grades. A short-course of anti-inflammatory prednisone does not appear to alter the histological parameters that affect grade-determination, nor significantly alter the proliferation indices in canine MCTs.