We resonantly inject spin-polarized field-emitted electrons in thermally switching nanomagnets. A detailed lifetime analysis as a function of the spin-polarized emission current reveals that considerable Joule heating is generated, and spin-transfer torque results in a directed switching. A trend of higher switching efficiency per electron is observed with an increasing emission current, probably due to the excitation of Stoner modes. On a quasistable nanomagnet, a spin-polarized emission current in the low nA regime already triggers magnetization reversal, thereby demonstrating the high impact of hot-electron spins onto atomic-scale magnets.