The low-energy level structure of the odd-odd nucleus $^{126}\mathrm{Sb}$ has been investigated by studying the radioactive decay of ${10}^{5}$-yr $^{126}\mathrm{Sn}$, using Ge(Li) and Si(Li) detectors. Based on conversion-electron and $\ensuremath{\gamma}$-ray singles data, $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ and $\ensuremath{\gamma}\ensuremath{-}{e}^{\ensuremath{-}}$ prompt and delayed coincidence measurements, and complementary $\ensuremath{\gamma}$-ray energy and intensity balance arguments, a decay scheme for $^{126}\mathrm{Sn}$ has been deduced which accommodates all of the observed $\ensuremath{\gamma}$-ray transitions. The energies (keV), spins, and parities of the levels established in $^{126}\mathrm{Sb}$ are as follows: 0.0 [${8}^{\ensuremath{-}}$], 17.7 [${5}^{+}$], 40.4 [${3}^{\ensuremath{-}}$], 83.1 [${4}^{\ensuremath{-}}$, (${3}^{\ensuremath{-}}$)], 104.7 [${3}^{+}$], and 128.0 [${2}^{+}$]. The respective half-lives of these six states are: 12.4 day, 19.0 min, \ensuremath{\cong}11 s, 5.1 ns, 553 ns, and 78.0 ns, the latter four values having been determined in the present investigation. The level assignments are discussed within the framework of the single-particle shell model and in relation to the proposed assignments of low-lying energy levels of neighboring odd-odd antimony isotopes.RADIOACTIVITY $^{126}\mathrm{Sn}$ [from $^{235}\mathrm{U}(n,f)$]; measured ${E}_{\ensuremath{\gamma}}$, ${I}_{\ensuremath{\gamma}}$, ${I}_{\mathrm{ce}}$, $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$, $\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}$, and $e\ensuremath{-}\ensuremath{\gamma}$ coin, level ${T}_{\frac{1}{2}}'\mathrm{s}$; deduced ICC, $\ensuremath{\gamma}$ multipolarities, $^{126}\mathrm{Sb}$ levels, $J$, $\ensuremath{\pi}$; calculated $\ensuremath{\gamma}$-hindrance factors. Ge(Li), Si(Li) detectors; mass-separated source.