Density functional theory was used to study how transition metal doping could be used as a method for systematically fine-tuning the structure and activity of MoS2 catalysts. Through studying the hydrogen evolution reaction (HER) on the edge sites, the role of the metal dopant was determined to be in modifying the strength of sulfur binding on the edge, which determines hydrogen binding onto sulfur atoms on the edge through a negative linear scaling. A simple thermodynamic quantity, ΔGS, is thus identified, which allows for a description of both the stable structure and adsorption at the edge sites. This provides a descriptor-based framework for the rational design of new MoS2-type catalysts, where a metal dopant can be chosen to either strengthen or weaken the binding of key intermediates as desired. We also elucidate the unique coverage dependence of hydrogen binding, which explains why MoS2-type catalysts tend to have near-optimal hydrogen binding. These results are expected to be more general and easily extended to other reactions on other layered transition metal dichalcogenides. Besides confirming the high HER activity of previously studied MoS2 catalysts, we find 6 additional candidates that show marked improve- ment in hydrogen adsorption free energies over pristine MoS2.