<p>The electrical conductivity (<i>σ</i>) of composites varies significantly depending on matrix polymer even when identical conductive fillers are employed. Here we elucidate the governing parameter of the filler–polymer interaction and <i>σ</i> of composites. The <i>σ</i> of the AgPolymer composites, synthesized by dispersing silver flakes (AgFLs) in different elastomer, thermoplastic, or thermoset polymers, varies by more than an order of magnitude in spite of the identical filler concentration (35 vol%). Although elastic modulus (<i>E</i>, related to the bond stiffness) and fracture energy (<i>G</i>, corresponding to the energy absorbed during bond cleavage) offer some insights, they are insufficient to fully elucidate the filler–polymer interaction. The excessive interaction between the AgFLs and poly-vinyl alcohol leads to the deviation from the <i>σ</i>-<i>E</i> and <i>σ</i>-<i>G</i> relationships of other AgPolymer composites. Surprisingly, the surface energy (<i>γ</i>) of composites (20.4–41.7 mJ m⁻²), obtained by the van Oss-Good model, is found to govern the <i>σ</i> of composites (1,598.4–36,508.0 S cm⁻¹) without an outlier. The dispersive and polar components of the <i>γ</i> of AgFLs and polymer determine the filler–polymer interaction, AgFL dispersion, and <i>σ</i> of composites. The <i>σ</i> can also be predicted using the intrinsic <i>γ</i> of AgFLs and polymers, before synthesizing composites, which is useful for the future composite design.</p>