There has been an intense search in recent years for long-lived spin-polarized carriers for spintronic and quantum-computing devices. Here we report that spin-polarized quasiparticles in superconducting aluminium layers have surprisingly long spin lifetimes, nearly a million times longer than in their normal state. The lifetime is determined from the suppression of the aluminium's superconductivity resulting from the accumulation of spin-polarized carriers in the aluminium layer using tunnel spin injectors. A Hanle effect, observed in the presence of small in-plane orthogonal fields, is shown to be quantitatively consistent with the presence of long-lived spin-polarized quasiparticles. Our experiments show that the superconducting state can be significantly modified by small electric currents, much smaller than the critical current, which is potentially useful for devices involving superconducting qubits.