Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the cause of the COVID-19 pandemic that began in 2019. The coronavirus 3-chymotrypsin-like cysteine protease (3CLpro) controls replication and is therefore considered a major target for antiviral discovery. This study describes the evaluation of SARS-CoV-2 3CLpro inhibitors in a novel self-assembled monolayer desorption ionization mass spectrometry (SAMDI-MS) enzymatic assay. Compared with a traditional FRET readout, the label-free SAMDI-MS assay offers greater sensitivity and eliminates false positive inhibition from compound interference with the optical signal. The SAMDI-MS assay was optimized and validated with known inhibitors of coronavirus 3CLpro such as GC376 (IC50 = 0.060 μM), calpain inhibitors II and XII (IC50 ~20–25 μM). The FDA-approved drugs shikonin, disulfiram, and ebselen did not inhibit SARS-CoV-2 3CLpro activity in the SAMDI-MS assay under physiologically relevant reducing conditions. The three drugs did not directly inhibit human β-coronavirus OC-43 or SARS-CoV-2 in vitro, but instead induced cell death. In conclusion, the SAMDI-MS 3CLpro assay, combined with antiviral and cytotoxic assessment, provides a robust platform to evaluate antiviral agents directed against SARS-CoV-2. •A novel label-free SARS-CoV-2 3CLpro enzymatic assay was developed using mass spectrometry (SAMDI-MS).•Protease inhibitors active in the SAMDI-MS assay under reducing conditions also have antiviral effects in cells.•Shikonin, disulfiram, and ebselen do not inhibit SARS-CoV-2 3CLpro in the SAMDI-MS assay.•Shikonin, disulfiram, and ebselen do not directly inhibit SARS-CoV-2 but instead are toxic to cells.