Nanocomposites composed of silver sulfide (Ag2S) and platinum (Pt) metal are promising for electrocatalysis. However, they suffer from low concentration synthesis or larger sizes due to limitations in the current synthetic approaches. Herein, we report an organic synthesis to address these deficiencies for the preparation of Ag2S-Pt nanocomposites. This strategy starts with a dodecylamine (DDA)-based phase transfer of Ag+ ions from the aqueous phase to toluene. The transferred Ag+ ions are then loaded on the carbon substrates, followed by reaction with elemental sulfur to form carbon-supported Ag2S nanocrystals, which are used as seeds for the growth of a Pt metal shell, resulting in the formation of the Ag2S-Pt nanocomposites on the same carbon substrates. The as-prepared carbon-supported Ag2S-Pt nanocomposites have a core-shell construction with an overall fine size of ca. 5 nm. In comparison with the commercial Pt/C catalysts from Johnson Matthey, fine Ag2S-Pt nanocomposites supported on the carbon substrates exhibit a superior specific activity and durability for the methanol oxidation reaction under acidic conditions due to the strong electronic coupling effect between the Ag2S and Pt domains.