Histone deacetylase (HDAC) inhibitors are employed for the treatment of lymphoma and are under development against multiple other types of cancer and neurodegenerative diseases. Here, we describe a robust and uncomplicated in vitro assay for HDAC inhibitor kinetic profiling. Enzyme and fluorogenic peptide substrate are incubated together with a small amount of protease “assay developer”, which enables continuous recording of substrate conversion under steady-state conditions. Assay progression curves upon addition of an inhibitors at varying concentrations permit determination of kinetic constants and overall inhibitor potency. This assay helped provide new insight into the kinetic properties of known HDAC inhibitors as well as the kinetic characterization of both inhibitors and substrates of sirtuin enzymes, which are class III HDACs involved in metabolic control and oncogene regulation.