The use of immobilized capillary enzyme reactors (ICERs) for online ligand screening has been adopted as a new technique for high-throughput screening (HTS). In this work, the selected target was the enzyme acetylcholinesterase (AChE), and the AChE-ICERs produced were used in a liquid chromatograph–tandem ion-trap mass spectrometer. The activity and kinetic parameters were evaluated by monitoring the choline’s precursor ion (M + H)+ m/z 104.0 and its ion fragment (C2H3OH) – (M + H)+ m/z 60.0. The assay method was validated using the reference AChE inhibitors tacrine and galanthamine. Two new ligands, out of a library of 17 coumarin derivatives, were identified, and the half-maximal inhibitory concentration (IC50), inhibition constant (K i), and the inhibition mechanism were determined. A coumarin derivative with IC50 similar to tacrine was highlighted.