The quasiparticle effective mass $m^{*}$ of the three-dimensional uniform electron gas is a fundamental Fermi-liquid parameter whose value and density dependence have remained controversial for decades. Using renormalized perturbation theory with explicit counterterms, we determine $m^{*}$ in the metallic regime ($r_s\le 6$) from first principles by two complementary routes—the self-energy and the forward-scattering four-point vertex via the $p$-wave spin-symmetric Landau parameter $F_1^s$—that agree within uncertainties at each density through sixth renormalized order. The resulting $m^{*}/m$ remains close to unity throughout the metallic regime, with a shallow non-monotonic density dependence—a minimum near $r_s\approx 1$ followed by a gentle upturn—reflecting the interplay of exchange and dynamical screening in the self-energy, and disfavoring strong monotonic suppression. This near-unity behavior is traced to the structural dominance of long-range charge fluctuations, whose isotropy in angular-momentum space suppresses $F_1^s$ that controls $m^{*}/m$.