Assessing target occupancy is critical for establishing proof-of-mechanism for novel inhibitors and to determine whether robust target inhibition can be achieved at tolerated doses. This is challenging in the clinic using conventional methods due to the need for untreated controls. We describe a new mass spectrometry approach to quantitatively assess target occupancy for covalent inhibitors that does not require untreated controls, and apply the method to the KRASG12C inhibitor ARS-1620.