We modify the fundamental electronic properties of metallic (1T phase) nanosheets of molybdenum disulfide (MoS2) through covalent chemical functionalization, and thereby directly influence the kinetics of the hydrogen evolution reaction (HER), surface energetics, and stability. Chemically exfoliated, metallic MoS2 nanosheets are functionalized with organic phenyl rings containing electron donating or withdrawing groups. We find that MoS2 functionalized with the most electron donating functional group (p-(CH3CH2)2NPh-MoS2) is the most efficient catalyst for HER in this series, with initial activity that is slightly worse compared to the pristine metallic phase of MoS2. The p-(CH3CH2)2NPh-MoS2 is more stable than unfunctionalized metallic MoS2 and outperforms unfunctionalized metallic MoS2 for continuous H2 evolution within 10 min under the same conditions. With regards to the entire studied series, the overpotential and Tafel slope for catalytic HER are both directly correlated with the electron donating strength of the functional group. The results are consistent with a mechanism involving ground-state electron donation or withdrawal to/from the MoS2 nanosheets, which modifies the electron transfer kinetics and catalytic activity of the MoS2 nanosheet. The functional groups preserve the metallic nature of the MoS2 nanosheets, inhibiting conversion to the thermodynamically stable semiconducting state (2H) when mildly annealed in a nitrogen atmosphere. We propose that the electron density and, therefore, reactivity of the MoS2 nanosheets are controlled by the attached functional groups. Functionalizing nanosheets of MoS2 and other transition metal dichalcogenides provides a synthetic chemical route for controlling the electronic properties and stability within the traditionally thermally unstable metallic state.