We present a minimal five-dimensional (5D) scalar field model in which the electron mass, the gravitational sector, Standard Model fermion masses, and cosmological dynamics all emerge from a single geometric mechanism. The model compactifies spacetime as ℝu2074 × S¹, where the compactification radius Ru2085 = Ru2080·e^Φ(x) is dynamically determined by a scalar field Φ. With only Newton’s constant G and the electron mass mu2091 as input, the framework predicts charged-lepton masses from a harmonic KK tower, reproduces the full quark spectrum using two flavour-blind parameters, and explains neutrino masses and mixing via a geometric seesaw. The same scalar field modifies gravity at galactic scales, reproducing observed rotation curves without dark matter. At cosmological scales, it drives inflation, reheating, baryogenesis, and late-time acceleration, with no new fields or fine-tuning. All predictions fall within current or near-future experimental reach, offering a falsifiable unification of mass generation, modified gravity, and cosmic evolution through geometry alone.