Functionalizing nanosheets of metallic molybdenum disulfide (MoS₂) provides a synthetic chemical route for controlling the electronic properties and stability within the traditionally thermally unstable metallic state. Metallic MoS₂ nanosheets have been studied for HER due to their higher reactivity for HER, earth abundance, low-cost, and non-toxicity, which makes metallic MoS₂ a candidate to replace platinum for HER. The functionalized nanosheets are more stable to the thermally initiated phase transformation from the metallic 1T phase to the semiconducting 2H phase. Furthermore, we show for an exemplary functionalized sample (Et₂NPh-MoS₂) that functionalization leads to better stability and long-term performance under HER conditions. These results provide a framework for understanding and controlling the balance between catalytic activity and stability for these unique 2D materials. Chemically-exfoliated, metallic (1T) 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 similar to the pristine metallic phase of MoS­­^­­­­₂. 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 pendant group on the phenyl ring. 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 sheet. We show that the functional groups preserve the metallic feature of the MoS₂ films, inhibiting conversion to the thermodynamically stable semiconducting state (2H) when annealed at 150 °C for 24 h in a nitrogen atmosphere. We propose that this protection is critical to maintaining the catalytically active state of 1T MoS₂ nanosheets. Our results demonstrate a strong correlation between the electron donating strength of substituents and the surface energetics, electron transfer resistance, and the HER catalytic activity of functionalized MoS₂ nanosheets.