Squamous cell carcinoma in situ (SCCIS) of the nail unit is a complex malignancy; with little understanding of rate of upstaging or occult invasion in these patients. We sought to evaluate the rate of upstaging in nail unit SCCIS after Mohs micrographic surgery (MMS). Retrospective review of 346 patients who referred for and underwent MMS for biopsy proven SCCIS on the hands and feet between January 1, 2000 and December 30, 2019. Only cases in the nail unit were included. Clinical, surgical details, histopathological features, HPV status, and rate of upstaging were recorded. Thirty-one cases met the inclusion criteria and were analyzed. Twenty-four patients were males (77.4%). The mean age was 55 years (SD 17.26, range 27-84). The mean clinical size was 9.9 mm; 19 cases tested for HPV, 15/19 were positive (78.9%), and 8/19 (42.1%) were associated with high-risk HPV. Three patients (9.7%) were upstaged to invasive on either MMS margins or tumor debulking. Limitations included a relatively small sample size and retrospective in nature. The rate of upstaging of SCCIS in the nail unit is not frequent, and when upstaging occurred it was focal, superficial, and with no PNI or bone invasion.